Run unit tests against PHP 8.4 alpha

[swissspidy]

Summary

Run tests against PHP 8.4 to see if our plugin still works as expected.

So far it looks good; all the deprecation warnings are coming from elsewhere.

Blocked by:

• https://github.com/ampproject/amp-toolbox-php/issues/549
• https://core.trac.wordpress.org/ticket/60786

User-facing changes

Testing Instructions

• [x] This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

Reviews

Does this PR have a security-related impact?

Does this PR change what data or activity we track or use?

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Checklist

• [ ] This PR addresses an existing issue and I have linked this PR to it
• [x] I have tested this code to the best of my abilities
• [ ] I have verified accessibility to the best of my abilities (docs)
• [ ] I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• [ ] This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• [ ] I have added documentation where necessary
• [x] I have added a matching Type: XYZ label to the PR

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Fixes #