PHP 8.5 needs to be added to the PHPUnit test matrix

[westonruter]

Feature Description

Now that WordPress 6.9 has beta support for PHP 8.5, we need to make sure that the Performance Lab feature plugins are also being tested with PHP 8.5.

The test matrix should only test WordPress 6.9 or trunk with PHP 8.5, since testing older versions of WP will likely cause warnings.

See #2095 which added PHP 8.4.

[westonruter]

Ah, there's a drafted PR already open for this: #2227