Use PHP-Code-Style 2.0

[jrfnl]

PHPCS: use PHP-Code-Style 2.0

• Composer: require php-parallel-lint/php-code-style at 2.0 or higher.
• Composer: remove the requirement for PHPCS. This will now be inherited from the code style repo.
• Composer: allow execution of the PHPCS Composer plugin, which is a dependency of the PHP Code Style repo.
• GH Actions, test workflow: adjust the removal of the PHPCS dependency to match.

Ruleset: update for PHP-Code-Style 2.0

• Use the ruleset from PHP-Code-Style, but keep the custom property settings.
• Set the testVersion for use with PHPCompatibility.
• Improve the documentation in the ruleset.

Ruleset: add a few selective exclusions

... for things to be addressed at a later point in time and a few which don't need addressing at all.

CS: various minor fixes

Fix up the issues which will be flagged by PHP-Code-Style 2.0 and explicitly ignore a couple of (non-)issues.

[grogy]

I prefer fix incorrect code with changing ruleset, but it is my personal preference. Thank you, merged :-)

[jrfnl]

@grogy Most issues flagged by the new ruleset have been fixed, I just excluded some which are more easily fixed in the "test refactor" project (and I have commits lined up to address all of those, except the cyclomatic complexity ones)