Recreate feature flags for merge trains

Merged Shinya Maeda requested to merge recreate-ff-for-merge-trains into master

What does this MR do?

As the context of #200037 (closed), we need a way to disable merge trains on a particular project for dogfooding Pipelines for merged results.

We also remove merge_trains_enabled feature flag as the feature itself is confirmed as deemed stable.

To disable Merge Trains on a particular project, please execute /chatops run feature set disable_merge_trains --project=gitlab-org/gitlab true

Screenshots

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • [-] Label as security and @ mention @gitlab-com/gl-security/appsec
  • [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • [-] Security reports checked/validated by a reviewer from the AppSec team
Edited by Shinya Maeda