Migrate contracts to Solidity 0.7 (#2319)

* Update contract pragmas to solidity 0.7

* Remove internal declaration on constructors

* Reference SafeMath explicitely

* Remove public constructor declaration from abstract contracts

* Remove public constructor declaration from non-abstract contracts

contracts/mocks/AccessControlMock.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../access/AccessControl.sol";
 
 contract AccessControlMock is AccessControl {
-    constructor() public {
+    constructor() {
         _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
     }