Prevent merge if the merge request pipeline is stale

What does this MR do?

When a project is enabling "Merge request pipelines" option, merge request pipelines will be generated in order for validating post-merge pipeline result. This is intended for preventing master branch from having red pipelines, however, currently users can merge MRs even though the target branch is advanced.

In this case, frontend shows the following warning, however, backend doesn't prevent merging at the moment.

We should not let users to continue merge process if it's the case.

What are the relevant issue numbers?

Close: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

This MR has CE backport

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26975
• EE: Here

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer