Address initial feedback

[pwtyler]

Got some initial feedback on the action a while back when Kevin initially spun up the project. To review and act on individually.

• action.yml: I'd recommend not setting a default for the test-versions, but to default to the same behaviour as PHPCompatibility has natively (second bullet in https://github.com/PHPCompatibility/PHPCompatibility#sniffing-your-code-for-compatibility-with-specific-php-versions)
• Sniffing for 8.0- while using PHPCompatibility 9.3 is kind of useless as 9.3 doesn't really contain any PHP 8 checks yet.
• entrypoint.sh: Composer install is not being cached
• Q: how does the composer install play with project native composer installs ?
• composer.json: please don't declare PHPCS as requirement, but let PHPCompatibility and the Composer PHPCS plugin handle that.
• Heads-up: as of PHPCompat 10.0, the Composer PHPCS plugin will be a dependency of PHPCompatibility, so no need to require it anymore. (you do still need to allow the plugin)
• Committed composer.lock: this could potentially be problematic if the setup PHP step is being skipped and the PHP version on which the composer install is being run does not match a supported version. Chances are slim as all three dependencies still support PHP 5.4, but still.

PHPCompatibility/PHPCompatibility#1414 (comment)