[Feature flag] Cleanup `use_received_header_for_incoming_emails`

Summary

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

This feature flag has been in production since %14.9 with !81489 (merged).

See gitlab-com/gl-infra/production#6419 (closed) for more context.

Owners

• Team: ~"group:certify"
• Most appropriate slack channel to reach out to: #f_service-desk or #g_product-planning
• Best individual to reach out to: @cablett or @johnhope
• PM: @mjwood

Expectations

What might happen if this goes wrong?

As of this writing, this feature flag has been default_enabled for ~3 months, so removal of the feature flag should be low risk. In the case of any issues the MR to remove the feature flag can be reverted.

Cleaning up the feature flag

• Create a merge request to remove use_received_header_for_incoming_emails 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 use_received_header_for_incoming_emails --dev
  • /chatops run feature delete use_received_header_for_incoming_emails --staging
  • /chatops run feature delete use_received_header_for_incoming_emails
• Close this rollout issue.