Tests: Run PHPStan on PHP 8.0+ only

[n7studios]

Summary

PHPStan's static analysis now only works on PHP 8.0 and higher, due to WordPress' stubs including #[\SensitiveParameter]:

This results in syntax errors in PHP 7.4 and lower:

This PR resolves by running static analysis on PHP 8.0 and higher.
Coding standards are run on PHP 7.2 and higher, to ensure backward compatibility should some users be on extremely old versions of PHP.
End to end tests run have been running on PHP 8.1 since May 2025

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)