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

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

contracts/mocks/AccessControlMock.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity >=0.6.0 <0.8.0;
+pragma solidity ^0.8.0;
 
 import "../access/AccessControl.sol";
 
 contract AccessControlMock is AccessControl {
-    constructor() public {
+    constructor() {
         _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
     }