Issue #3573464: Change testing_package_manager requirement severity to Warning

After installing Drupal 11.3 via CMS 2, admin/reports/status reports this error:

Errors Found: Package Manager Package Manager is available for early testing. To install the module set the value of 'testing_package_manager' to TRUE in your settings.php file

In Drupal CMS the Package Manager module is enabled by a recipe.

This should not be an error - changed requirement severity from Error to Warning