Pin solidity version (#2)

Author: mdehoog

src/Paymaster.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0
-pragma solidity ^0.8.12;
+pragma solidity 0.8.20;
 
 /* solhint-disable reason-string */
 /* solhint-disable no-inline-assembly */