Upgrade Psalm to 6.7 and silence issues

[kamil-tekiela]

This PR upgrades Psalm and silences all the newly enabled nonsense checks introduced in vimeo/psalm#11283:

• Final classes. While most agree that classes should be final by default, our testing scenario demands that we allow classes to be doubled.
• Override attribute. I don't see much value in this attribute. It would have to be added to many files for little benefit. All it does is help find potentially dead code, which static analysers and IDEs can already do.
• Strict binary operands. This one just seems plain wrong to me. PHP can happily cast the operands into the required type without causing any issues. echo 'Hello' . 42; is perfectly fine in my opinion. Suppressing this check would require scattering explicit type casts all over the code. This really shouldn't be an issue reported by static analysis. Maybe I am missing something here, but I couldn't find any justification for this one.

These checks can be enabled later on in separate PRs, but for the sake of a seamless Psalm upgrade let's keep them disabled as they previously were.