Provide instance level setting to enable or disable "default branch protection" at the group level for group owners

Follow up to: !24426 (merged)

Proposal:

Have an instance level setting that instance admins can enable or disable.

When enabled: Allow group owners to update the default branch protection setting at the group level. Instance admins should also be able to update this setting at the group level.

When disabled: Do not allow group owners to update the default branch protection setting at the group level. But Instance admins should still be able to update this setting at the group level.