Reset merge status from mergeable MRs

What does this MR do?

Needed for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28513. This will reset opened, mergeable MRs to a unchecked state. That means the new MergeabilityCheckService will update both the merge_status and merge-ref altogether.

This is required mainly to avoid a can_be_merged MR to stick with an outdated merge-ref.

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