[Feature flag] Cleanup `use_namespace_historical_statistics_for_group_security_dashboard`

Summary

This issue is to cleanup the use_namespace_historical_statistics_for_group_security_dashboard feature flag after the feature flag has been enabled by default for an appropriate amount of time in production. This feature will be completely removed.

This feature flag was introduced to avoid breaking the functionality of the Group level security dashboard for self-managed instances. Since most of the dashboard data is generated through a backfill migration (using existing Project level statistics), the plan was to gradually roll out the change and remove the feature flag once we can assume the backfill process has been completed for self-managed instances.

See !166139 (comment 2232206597)

Owners

• Team: SRM:Security Platform Management
• Most appropriate slack channel to reach out to: #g_srm-security-platform-management
• Best individual to reach out to: @gkatz1

Expectations

What might happen if this goes wrong?

Removing this feature flag too early might cause the Security Dashboard for a Group level to show partial data.

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.