Selectively disable DRAFT state merged results pipelines with `legacy_merge_request_state_check_for_merged_result_pipelines` feature flag

Summary

In GitLab 15.1, we changed the merged results pipelines to run on DRAFT MR states. This flag is for switching to a previous behavior on a specific project if a user requested. If no one requests to use the previous behavior, we'll never enable this flag. We'll remove this flag in 16.0 and still keep the new behavior as a permanent change.

For more details, see !91849 (merged)

This MR introduces a feature flag to selectively disable the fixed behavior.

Originally, we went through a fix and feature flag rollout process in !86612 (merged) and !89951 (merged), however, it seems there are still a few users that needs time to adjust their workflow. For those users, we can restore the previous behavior on specific projects so that they can buy more time to adjust their workflow.

Owners

• Team: grouppipeline execution
• Most appropriate slack channel to reach out to: g_pipeline_execution
• Best individual to reach out to: @shinya.maeda
• PM: @jheimbuck_gl

Expectations

What are we expecting to happen?

It runs detached merge request pipelines on DRAFT MR, instead of merged result pipelines.

When is the feature viable?

Merged results pipelines is enabled on the project.

What might happen if this goes wrong?

This feature has already been globally rolled out in GitLab 15.1.

What can we monitor to detect problems with this?

#247067 (closed) is the central issue to detect user feedback.

What can we check for monitoring production after rollouts?

#247067 (closed) is the central issue to detect user feedback.

Selectively disable per user demand

If a user requested, you can run the following command:

• /chatops run feature set --project=<project-path> legacy_merge_request_state_check_for_merged_result_pipelines true