When recheck flag is true, we make sure the merge-ref
is indeed outdated. If it is, we update it along
the merge status.

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.

When target branch is updated, Auto Merge should be canceled.

We don't have a way to create MR pipeline today, this is one
of the headaches for users.
This change is a preliminary work for the capability.

We should cancel auto merge when merge request is closed.

Automatically update MR merge-ref along merge status

Closes #58495

See merge request gitlab-org/gitlab-ce!28513

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

This introduces payload to the ServiceResponse with
the merge ref HEAD commit data