Improve PHP 8.4 compatibility

[swissspidy]

Summary

This addresses all deprecation warnings on PHP 8.4, unblocking PHP 8.4 compatibility in the Web Stories plugin.

The handful of PHPStan failures are unrelated and can be addressed separately.

There are some test failures too but they can be handled separately as well.

Checklist

• 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).

[westonruter]

Regarding the PHPStan issues, see also #7876.

[westonruter]

@thelovekesh Could you address the remaining PHPStan issues?

[westonruter]

As part of this, we should also update PHP-CSS-Parser which was just updated to improve PHP 8.4 compatibility: https://github.com/MyIntervals/PHP-CSS-Parser/releases/tag/v8.7.0

[swissspidy]

Might be a bigger change that warrants its own PR, as I assume the package‘s namespace changed as well and all those parches don‘t apply anymore

[westonruter]

The remaining error is due to a change in PHP 8.4 for the string representation of a closure. Should be a trivial fix.

[swissspidy]

I'll defer that to you then 😃

[westonruter]

Oof. That was more of a headache than I was expecting! Really confused why I needed to do d1e4a50 now just for PHP 8.4 although it also is failing for me locally in PHP 8.3. It should have been failing for all PHP versions.