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

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

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";