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

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

contracts/mocks/PullPaymentMock.sol

@@ -1,12 +1,12 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity >=0.6.0 <0.8.0;
+pragma solidity ^0.8.0;
 
 import "../payment/PullPayment.sol";
 
 // mock class using PullPayment
 contract PullPaymentMock is PullPayment {
-    constructor () public payable { }
+    constructor () payable { }
 
     // test helper function to call asyncTransfer
     function callTransfer(address dest, uint256 amount) public {