Allow compilation with later solidity versions

[jhuntbach-bc]

The v1 solidity files all have a pragma solidity 0.8.7 and v2 has pragma solidity 0.8.26. Could we add a ^ before the version numbers to allow compilation with later versions? This would allow other project to import these files and compile them with whatever latest version they're using, rather than having a constraint on using the exact version specified.

This comment suggests that the v2 contracts were meant to have a ^ anyway:
#156 (comment)