[Feature flag] Cleanup group_mention_access_check

Summary

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

Related: Slack Notification Integration information leak (#425067 - closed)

Owners

• Team: ProdSecEng
• Most appropriate slack channel to reach out to: #prod-sec-eng-team-internal
• Best individual to reach out to: @ashmckenzie
• PM: N/A

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 group_mention_access_check 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 5053d9df
• [-] 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 group_mention_access_check --dev --pre --staging --staging-ref --production
• Close this rollout issue.