actions: Reenable experimental PHP 8.1 run

[anomiex]

Changes proposed in this Pull Request:

This will reenable tests against PHP 8.1, which had been disabled in #22850.

Jetpack product discussion

p1644842786633359-slack-CBG1CP4EN
p9dueE-4qH-p2

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Make sure that the PHP 8.1 tests are either passing or are failing because of errors in our own code rather than WordPress Core.
• Or make sure GitHub Actions will now show the failing test with some sort of "expected" status, as described in actions/toolkit#399, rather than various status indicators showing "failing".

[github-actions]

Thank you for your PR!

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[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run bin/jetpack-downloader test jetpack update/reenable-php-8.1-tests to get started. More details: p9dueE-5Nn-p2

[anomiex]

PHP 8.1 tests pass now!