PHP Compatibility Workflow doesn't require a matrix

Describe the bug

As the PHP Compatibility checker uses a tokenizer to evaluate compatibility (as opposed to code execution), it can be run on a single version of PHP rather than as part of a matrix.

The test version can be listed as 7.4- to test for compatibility with all supported versions of PHP.

This will require a change to the protected branch config to remove the redundant runs as required checks.

Steps to Reproduce

N/A

Screenshots, screen recording, code snippet

Docs: https://github.com/PHPCompatibility/PHPCompatibility#testversion-in-the-ruleset-versus-command-line

Environment information

No response

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