Do not display pipeline ref as link if it doesn't exist

What does this MR do?

Changes the display of the pipeline ref in the commit info section of the pipeline view and the pipelines list. Previously, if the ref did not exist, it would be displayed as a link that would 404. This MR changes it to display as text (span tag with ref-name class).

Adds the exists field to the ref property of the pipeline serializer.

Increases the gitaly requests in the pipeline controller spec from 3 to 4.

Closes #42611 (closed).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See #42611 (closed).

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

42611