Refactor checking :enforce_locked_labels_on_merge feature flag

What does this MR do and why?

Refactors checking the :enforce_locked_labels_on_merge feature flag. Also ensure the flag is valid for the project and it's ancestors (meaning you can set the flag on a group and all subgroups/projects will have it enabled)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #408676