add php 8.5 tests for latest and trunk version of WP

[masteradhoc]

Fixes #768

What?

Extend the GitHub Actions test matrix to include PHP 8.5, while excluding it for WordPress 6.8 where core support is not available.

Why?

How?

PHP 8.5 is now a released PHP version and should be covered by CI to detect early compatibility issues.
WordPress core support for PHP 8.5 begins with WordPress 6.9, so running PHP 8.5 against the latest trunk and stable would be beneficial.

Testing Instructions

Open the GitHub Actions run for this PR.

Verify that:

1. PHP 8.5 runs with latest (6.9) and trunk.
2. PHP 8.5 does not run with WordPress 6.8.
3. Confirm all remaining matrix jobs complete successfully.

Screenshots or screencast

Changelog Entry

Changed – CI now includes PHP 8.5 testing, excluding unsupported WordPress 6.8 combinations.

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[kasparsd]

Great to have these tests extended!