Update contracts to support Solidity 0.8.x (#2442)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

docs/modules/ROOT/pages/access-control.adoc

@@ -13,7 +13,7 @@ OpenZeppelin provides xref:api:access.adoc#Ownable[`Ownable`] for implementing o
 ----
 // contracts/MyContract.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/Ownable.sol";
 
@@ -62,7 +62,7 @@ Here's a simple example of using `AccessControl` in an xref:tokens.adoc#ERC20[`E
 ----
 // contracts/MyToken.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
@@ -94,7 +94,7 @@ Let's augment our ERC20 token example by also defining a 'burner' role, which le
 ----
 // contracts/MyToken.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
@@ -139,7 +139,7 @@ Let's take a look at the ERC20 token example, this time taking advantage of the
 ----
 // contracts/MyToken.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
@@ -148,7 +148,7 @@ contract MyToken is ERC20, AccessControl {
     bytes32 public constant MINTER_ROLE = keccak256("MINTER_ROLE");
     bytes32 public constant BURNER_ROLE = keccak256("BURNER_ROLE");
 
-    constructor() public ERC20("MyToken", "TKN") {
+    constructor() ERC20("MyToken", "TKN") {
         // Grant the contract deployer the default admin role: it will be able
         // to grant and revoke any roles
         _setupRole(DEFAULT_ADMIN_ROLE, msg.sender);

docs/modules/ROOT/pages/erc1155.adoc

@@ -34,7 +34,7 @@ Here's what a contract for tokenized items might look like:
 ----
 // contracts/GameItems.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
 
@@ -45,7 +45,7 @@ contract GameItems is ERC1155 {
     uint256 public constant SWORD = 3;
     uint256 public constant SHIELD = 4;
 
-    constructor() public ERC1155("https://game.example/api/item/{id}.json") {
+    constructor() ERC1155("https://game.example/api/item/{id}.json") {
         _mint(msg.sender, GOLD, 10**18, "");
         _mint(msg.sender, SILVER, 10**27, "");
         _mint(msg.sender, THORS_HAMMER, 1, "");
@@ -132,7 +132,7 @@ In order for our contract to receive ERC1155 tokens we can inherit from the conv
 ----
 // contracts/MyContract.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC1155/ERC1155Holder.sol";
 

docs/modules/ROOT/pages/erc20-supply.adoc

@@ -14,7 +14,7 @@ Let's say we want a token with a fixed supply of 1000, initially allocated to th
 [source,solidity]
 ----
 contract ERC20FixedSupply is ERC20 {
-    constructor() public {
+    constructor() {
         totalSupply += 1000;
         balances[msg.sender] += 1000;
     }
@@ -26,7 +26,7 @@ Starting with Contracts v2 this pattern is not only discouraged, but disallowed.
 [source,solidity]
 ----
 contract ERC20FixedSupply is ERC20 {
-    constructor() public ERC20("Fixed", "FIX") {
+    constructor() ERC20("Fixed", "FIX") {
         _mint(msg.sender, 1000);
     }
 }
@@ -44,7 +44,7 @@ The mechanism we will implement is a token reward for the miners that produce Et
 [source,solidity]
 ----
 contract ERC20WithMinerReward is ERC20 {
-    constructor() public ERC20("Reward", "RWD") {}
+    constructor() ERC20("Reward", "RWD") {}
 
     function mintMinerReward() public {
         _mint(block.coinbase, 1000);
@@ -92,7 +92,7 @@ Adding to the supply mechanism from previous sections, we can use this hook to m
 [source,solidity]
 ----
 contract ERC20WithAutoMinerReward is ERC20 {
-    constructor() public ERC20("Reward", "RWD") {}
+    constructor() ERC20("Reward", "RWD") {}
 
     function _mintMinerReward() internal {
         _mint(block.coinbase, 1000);

docs/modules/ROOT/pages/erc20.adoc

@@ -15,7 +15,7 @@ Here's what our GLD token might look like.
 ----
 // contracts/GLDToken.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 

docs/modules/ROOT/pages/erc721.adoc

@@ -14,7 +14,7 @@ Here's what a contract for tokenized items might look like:
 ----
 // contracts/GameItem.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
 import "@openzeppelin/contracts/utils/Counters.sol";
@@ -23,7 +23,7 @@ contract GameItem is ERC721 {
     using Counters for Counters.Counter;
     Counters.Counter private _tokenIds;
 
-    constructor() public ERC721("GameItem", "ITM") {}
+    constructor() ERC721("GameItem", "ITM") {}
 
     function awardItem(address player, string memory tokenURI)
         public

docs/modules/ROOT/pages/erc777.adoc

@@ -20,7 +20,7 @@ We will replicate the `GLD` example of the xref:erc20.adoc#constructing-an-erc20
 ----
 // contracts/GLDToken.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC777/ERC777.sol";
 

docs/modules/ROOT/pages/extending-contracts.adoc

@@ -20,7 +20,7 @@ For example, imagine you want to change xref:api:access.adoc#AccessControl[`Acce
 ```solidity
 // contracts/ModifiedAccessControl.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 
@@ -48,7 +48,7 @@ Here is a modified version of xref:api:access.adoc#AccessControl[`AccessControl`
 ```solidity
 // contracts/ModifiedAccessControl.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/access/AccessControl.sol";
 
@@ -80,7 +80,7 @@ Hooks are simply functions that are called before or after some action takes pla
 Here's how you would implement the `IERC721Receiver` pattern in `ERC20`, using the xref:api:token/ERC20.adoc#ERC20-_beforeTokenTransfer-address-address-uint256-[`_beforeTokenTransfer`] hook:
 
 ```solidity
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 

docs/modules/ROOT/pages/gsn-strategies.adoc

@@ -108,7 +108,7 @@ Your GSN recipient contract needs to inherit from `GSNRecipientERC20Fee` along w
 ----
 // contracts/MyContract.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/GSN/GSNRecipientERC20Fee.sol";
 import "@openzeppelin/contracts/access/AccessControl.sol";
@@ -116,11 +116,11 @@ import "@openzeppelin/contracts/access/AccessControl.sol";
 contract MyContract is GSNRecipientERC20Fee, AccessControl {
     bytes32 public constant MINTER_ROLE = keccak256("MINTER_ROLE");
 
-    constructor() public GSNRecipientERC20Fee("FeeToken", "FEE") {
+    constructor() GSNRecipientERC20Fee("FeeToken", "FEE") {
         _setupRole(MINTER_ROLE, _msgSender());
     }
 
-    function _msgSender() internal view override(Context, GSNRecipient) returns (address payable) {
+    function _msgSender() internal view override(Context, GSNRecipient) returns (address) {
         return GSNRecipient._msgSender();
     }
 
@@ -154,7 +154,7 @@ Once your strategy is ready, all your GSN recipient needs to do is inherit from
 [source,solidity]
 ----
 contract MyContract is MyCustomGSNStrategy {
-    constructor() public MyCustomGSNStrategy() {
+    constructor() MyCustomGSNStrategy() {
     }
 }
 ----

docs/modules/ROOT/pages/index.adoc

@@ -28,7 +28,7 @@ Once installed, you can use the contracts in the library by importing them:
 ----
 // contracts/MyNFT.sol
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.6.0;
+pragma solidity ^0.8.0;
 
 import "@openzeppelin/contracts/token/ERC721/ERC721.sol";