Tests: allow for PHPUnit 8/9

[jrfnl]

Follow up on #805

PHP_CodeSniffer 3.8.0 now allows for running the tests, which are based on the PHPCS native test suite, with PHPUnit 8 and 9.

This commit updates the package to take advantage of that.

Includes:

• Widening the PHPUnit version requirements.
• Adding the PHPUnit 8+ cache file to .gitignore.
• Updating the PHPUnit configuration to make sure that PHP deprecations will always show, even when on a high PHPUnit 9.6 version.
• Updating the info in CONTRIBUTING.
• Simplifications to the quicktest and test workflows. Also, the code coverage "high" run can now be run against PHP 8.3.
• Running PHPStan against PHP latest (couldn't previously be done due to the old PHPUnit version).
• Updating the unit-tests script to remove the PHP version based toggle, which is no longer needed.

Ref:

• PHPCSStandards/PHP_CodeSniffer#59