Correct permissions entry on deleting protected branch

What does this MR do?

Corrrects the permissions table entry on deleting a protected branch. The documentation currently states that no-one can do so. In fact, anyone with at least Maintainer permissions can delete a protected branch, but only using either the UI or API.

Note: I consulted Ash McKenzie, Staff Backend Engineer in the Source Code group to verify the permissions.

After doing some local testing, Ash replied:

I've confirmed that Maintainer+ can delete a protected branch via the UI / API but not via the command line, e.g. git push origin --delete :

$ git push origin --delete test-protected-delete
remote: GitLab: You can only delete protected branches using the web interface.
To ssh://gdk.test:2222/root/lfs-mirror-test.git
 ! [remote rejected] test-protected-delete (pre-receive hook declined)
error: failed to push some refs to 'ssh://gdk.test:2222/root/lfs-mirror-test.git'

Related issues

N/A

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