WIP: Create `merge-requests/train` ref and run merge request pipelines on it

What does this MR do?

Previously, merge request pipelines are a run on refs/merge refs, however, we cannot keep using this ref for Merge Train feature, because it's conflicted with ~Create team's future iteration.

We know using refs/merge refs for Merge Train simplifies the things a lot.

So let's use refs/train only. Everywhere. Regardless of its merge request pipelines or merge train pipelines. We can create a new ref from a previous merge request's refs/merge and the current source sha, when we've done gitaly!1210 (merged).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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