What?

This PR:

• Removes PHP related checks from the Unit Tests Github CI job;
• Adds 2 new Github CI Jobs: PHPUnit Tests and PHP Coding Standards (the same way it's implemented in Core);
• brings ability to run PHPUnit tests on all PHP versions (5.6 through 8.2) using the CI jobs;
• shows code style errors on the "Files changed" tab (same as Core);
• Fixes an issue with wp-env as it used non-optimal PHPUnit versions on PHP 7.3 and 7.4 (props to @jrfnl);

Why?

Gutenberg should support the same PHP versions as Core.

How?

This PR adds 2 new Github CI Jobs: PHPUnit Tests and PHP Coding Standards.
These PHP CI jobs were backported from Core to ensure that we test Gutenberg PRs in the same fashion.

Testing Instructions

1. Make sure that the CI jobs run without errors.
2. Observe there are PHP_VERSION (multisite) on ubuntu-latest jobs for each supported PHP version (5.6 through 8.2).

Props @jrfnl , @dmsnell , @anton-vlasenko