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

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

contracts/GSN/Context.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity >=0.6.0 <0.8.0;
+pragma solidity ^0.8.0;
 
 /*
  * @dev Provides information about the current execution context, including the
@@ -13,7 +13,7 @@ pragma solidity >=0.6.0 <0.8.0;
  * This contract is only required for intermediate, library-like contracts.
  */
 abstract contract Context {
-    function _msgSender() internal view virtual returns (address payable) {
+    function _msgSender() internal view virtual returns (address) {
         return msg.sender;
     }