Fix MR pipelines run on only: regexp

What does this MR do?

The matching by pattern and ref specification makes no sense if the source in merge-requests.

This, if needed, has to be explicitly implemented as separate feature that extends the merge-requests matching. Likely in the form:

only:
  merge-requests:
    - source-ref-name

Making this work with so generic and messy refs: makes this not behave correctly. This ensures that we can ref-match only tags and branches. The MR pipelines are separate sub-entity of pipelines.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55026

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.
• Security reports checked/validated by reviewer