Run Tests against PHP 8.5

[n7studios]

Summary

Runs tests against PHP 8.5.
Drops tests for PHP 8.1, as this is end of life: https://www.php.net/supported-versions.php

Coding Standards and PHPStan continue to run against PHP 7.2 and higher to ensure backward compatibility is maintained.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)