Fix cache poisoning causing pipeline_schedule_policy_spec.rb to fail

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 spec/policies/ci/pipeline_schedule_policy_spec.rb to fail since the protected branch cache might have the wrong values.

To fix this particular issue, clean the Redis cache in the spec.

Closes #371929 (closed)

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