Quarantine flaky protected_branches_spec context

What does this MR do and why?

quarantine flaky test reported in Failure in ee/spec/features/protected_branches_... (#437794). The impacted context has caused at least 8 failing master pipelines since Janurary so I propose we quarantine the spec to restore stability. I have pinged @sean_carroll in the issue to investigate the flakiness, hopefully we can restore coverage soon.

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