Fix: GitHub workflows broken due to outdated versions / OS

[agibson-godaddy]

Summary

Updates the versions used for various actions to address:

Error: This request has been automatically failed because it uses a deprecated version of actions/cache: 937d24475381cd9c75ae6db12cb4e79714b926ed. Please update your workflow to use v3/v4 of actions/cache to avoid interruptions. Learn more: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

Also removes deprecated usage of ubuntu-20.04 -- see actions/runner-images#11101

Also updates the phpstan config to address an unrelated failure. See https://github.com/10up/wp_mock/pull/255/files#r1983301587

Details

Contributor checklist

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly
• I have added tests to cover changes introduced by this pull request
• All new and existing tests pass

Testing

• Code review
• Checks pass / GH actions run successfully

Reviewer checklist

• Code changes review
• Documentation changes review
• Unit tests pass
• Static analysis passes

[agibson-godaddy]

I made this change to address this error:

Seems like a good idea anyway. (it was throwing the error because a docblock typed it as a string, but that doesn't actually enforce the type in reality, which is why I think the config change makes sense)

[ajaynes-godaddy]

LGTM