[Feature flag] Cleanup `ci_fix_max_includes`

Summary

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

This FF was introduced in Fix to prevent unlimited number of CI includes (!112963 - merged) and successfully rolled out globally in #390909 (closed).

The change behind this FF fixes a bug that formerly allowed unlimited nested includes in a CI config. We decided to turn this FF to default ON prior to removal in order to ensure self-managed instances have a sufficient adjusting period.

Owners

• Team: Pipeline Authoring
• Most appropriate slack channel to reach out to: #g_pipeline_authoring
• Best individual to reach out to: @lma-git @furkanayhan
• PM: @dhershkovitch

Expectations

What might happen if this goes wrong?

It's unlikely that anything would go wrong by the time we implement this clean up since the Feature Flag would have been defaulted to ON for at least one milestone.

However, if there is an issue: We could roll back the clean up MR (!117954 (merged)), and turn off the feature flag ci_fix_max_includes.

Cleaning up the feature flag

• 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
  • /chatops run feature delete <feature-flag-name> --staging
  • /chatops run feature delete <feature-flag-name>
• Close this rollout issue.