This:
* Updates the minimum PHPCS requirement in `composer.json`, the `README` and the `CONTRIBUTING` docs.
    Includes removing some phrases which referenced use of older PHPCS versions.
* Updates the GH Actions scripts to no longer test against PHPCS < 3.7.1.
    Includes:
    - Removing a work-around which was in place for a dependency which isn't compatible with PHPCS 2.x.
    - Minor simplification for the `quicktest` script regarding whether or not to run linting.
* Updates the `LowPHPCS` sniff to reflect the new minimum and recommended PHPCS version.
    This sniff will now not trigger any errors anymore, however, I recommend leaving the sniff in place for now for future "resurrection".
* No need to include the `PHPCSUtils23` ruleset anymore (which added aliases for PHPCS 2.x classes).
* Add PHPCSDevCS to the Composer `require-dev` dependencies, now there is no longer a conflict with the minimum supported PHPCS version.
    Includes removing work-arounds for this from the Composer scripts and in the workflows.