PHP 8.5 Compatibility

[n7studios]

Summary

Fixes #1172, where a Using null as an array offset is deprecated PHP deprecation notice would display when using PHP 8.5 or higher.

This wasn't caught previously because:

• GitHub actions did not run against PHP 8.5,
• Xdebug wasn't configured to output PHP deprecated notices in GitHub actions

This PR:

• Resolves the above,
• Runs tests against PHP 8.5,
• Configures the GitHub actions runner to show PHP deprecations as Xdebug errors, ensuring checkNoWarningsAndNoticesOnScreen catches deprecation notices
• 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)

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[github-actions]

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