update test framework to PHPUnit 9 with polyfills

Development and testing of Statify becomes quite a pain on modern systems as compatibility gaps between PHP, PHPUnit and WP increase.

With WP 5.9 we got some handy polyfills for extended PHPUnit compatibility, s.t. we finally can use PHPUnit 9 while PHPUnit 7 won't even initialize on PHP 8.1

We add some dirty hacking here to leverage the polyfills with legacy WP 4.7 setup to preserve a case for the oldest supported version.

Update test matrix up to PHP 8.2 and WP 6.1.

Finally, we move to "ubuntu-latest" runners with dedicated MySQL setup action instead of deprecated 18.04 runners.

pluginkollektiv / statify

update test framework to PHPUnit 9 with polyfills #233