fixed: More flexible Solidity version declaration in BigNumbers.sol [#28] (#29)

Author: siriusyim

contracts/vendor/solidity-BigNumber/src/BigNumbers.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.8.18;
+pragma solidity ^0.8.18;
 
 /**
  * @title BigNumbers