Add feature flag allow_protected_branches_for_group

What does this MR do and why?

1. We disallow using a feature flag name which is same as the licensed feature name. Existing feature flag group_protected_branches clashed with the licensed feature name, therefore, adding another feature flag allow_protected_branches_for_group to resolve the conflict and allow rolling out the feature flag on production.
2. This resulted in preventing rolling out of the feature flag group_protectected_branches. Check the issue's description for more details.
3. Instead of renaming, added another feature flag and allow the feature to be used if either of the two feature flags are enabled.This will ensure that the existing feature flag still works for the self-managed customers who might have enabled the group_protected_branches feature flag via rails console on their instance. They will not have to re-enable the new feature flag.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable allow_protected_branches_for_group feature flag and ensure that group protected branches feature works.
2. Enable group_protected_branches feature flag and disable allow_protected_branches_for_group feature flag. Ensure that the group protected branches feature still works.

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.

Closes #404726 (closed)