Cherry-pick: Use latest PHPStan version to run its tests and replace deprecated autoload_files option

[westonruter]

Summary

Cherry picks commits from #4738 and #4868 into the 1.5 branch. Note that an intermediary PR #4867 was already cherry-picked.

Fixes #4745
Fixes #4865

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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