[Feature flag] Cleanup optimize_credentials_inventory

Summary

This issue is to cleanup the optimize_credentials_inventory feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.

• Finalize db/docs/batched_background_migrations/backfill_group_id_and_user_type_for_human_users_personal_access_tokens.yml and db/docs/batched_background_migrations/backfill_group_id_and_user_type_for_nonhuman_access_tokens.yml batched background migrations
• Ensure that credentials_inventory_pat_finder FF and all related code are removed.
  • Revert !193375 (merged)
  • Remove index_pats_on_user_id_and_created_at_and_pat_id, index_pats_on_user_id_and_expires_at_and_pat_id, and index_pats_on_user_id_and_last_used_at_and_pat_id indexes
• Suggest to clean up manage_pat_by_group_owners_ready FF.

Owners

• Team: groupauthentication
• Most appropriate slack channel to reach out to: ##g_sscs_authentication
• Best individual to reach out to: @bdenkovych
• PM: NAME

Stakeholders

Expectations

What might happen if this goes wrong?

Cleaning up the feature flag

• Specify in the issue description if this feature will be removed completely or will be productized as part of the Feature Flag cleanup
• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• If not already done, clean up the feature flag from all environments by running these chatops command in #production channel: /chatops run feature delete <feature-flag-name> --dev --pre --staging --staging-ref --production
• Close this rollout issue.