Add FF for SaaS GA release of Semantic Code Search

What does this MR do and why?

When checking for the eligibility to run Code Embeddings indexing for a namespace, we make use of the namespace_settings.experiment_features_enabled check. When we go to GA, this check will have to be dropped.

This change drops the namespace_settings.experiment_features_enabled check behind a new FF semantic_search_ga. This means we can simply enable the FF to roll out to GA, instead updating the code to remove the namespace_settings.experiment_features_enabled check.

This also allows for easier roll back.

Note for SM instances: The experiment features enabled/disabled state of an instance will create or delete records for all namespaces. I think it's better not to have an easy to toggle flag for SM as it will affect all namespaces.

References

• Issue for dropping experimental checks for GA: Duo Agent Platform - Remove Beta/Experimental c... (gitlab-org#19948)
• Related Issue specific to the Code Embeddings / Semantic Search feature: Introduce a GA rollout Feature Flag for Code Em... (#582909)
• FF rollout issue: [FF] `semantic_code_search_saas_ga ` -- for Sem... (#582911)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

Query Plans

Query in ProjectsFinder

• With namespace_settings.experiment_features_enabled check
• FF enabled: without namespace_settings.experiment_features_enabled check

Query in CreateEnabledNamespaceEventWorker & ProcessInvalidEnabledNamespaceEventWorker

• With namespace_settings.experiment_features_enabled check
  • MR introducing the query: Delete invalid enabled namespaces (!214831 - merged)
  • Query plan in MR
  • Query plan using the same IDs as below for comparison
• FF enabled: without namespace_settings.experiment_features_enabled check

MR acceptance checklist

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