Clear protected branch cache upon creation in test

What does this MR do and why?

The introduction of the protected branch caching via the hash_based_cache_for_protected_branches feature flag could cause tests to fail since the protected branch cache might have the wrong values.

Normally the cache is refreshed since protected branches are created with ProtectedBranches::CreateService, but in tests FactoryBot creates them without the service. To ensure the cache is in a good state in tests, mimic this cache clearing after the creation of a protected branch in the factory.

Relates to #371944 (closed)

How to set up and validate locally

Previously this test failed (https://gitlab.com/gitlab-org/gitlab/-/jobs/2948421534):

FOSS_ONLY=1 bundle exec rspec ./spec/controllers/projects/merge_requests_controller_spec.rb

MR acceptance checklist

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