BREAKING CHANGE: Much of the "setup" that's built-into the command is removed in favor of a leaner action. This also includes the "on PR, only diff PR contents" behavior. This can be restored, but it shouldn't be the default and should be done as an input.
Fix SEVERITY_FLAGS construction which exited 1 under bash -e when
severity inputs were empty, causing CI failures.
Add inputs to allow configuring error and warning severity levels.
This allows suppressing messages in the Mage-OS repository that are
ignored in upstream Magento Open Source CI, too.