actions: Disable experimental PHP 8.1 run

[anomiex]

Changes proposed in this Pull Request:

We enabled this back in October 2021 in the hope that WordPress Core
would quickly make progress on their own PHP 8.1 compatibility project.
That has not at all been born out, four months later they still need to
update their copy-pasted version of their own requests library and
still need to fix another random deprecation warning despite a short
PR existing.

GitHub's handling for expected-failing tests is lacking, so having a
test, even one flagged experimental, failing long term causes some other
issues. At this point, let's just disable the test until one or the
other makes some progress.

Jetpack product discussion

p1644842786633359-slack-CBG1CP4EN

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

no

Testing instructions:

• The "PHP tests: PHP 8.1 WP master" job should no longer run.

[github-actions]

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[anomiex]

For the record: p9dueE-4qH-p2