Allow dot in branch name in trigger/builds endpoint in API

What does this MR do?

Adds a requirement for the :ref param in the POST /projects/:id/ref/:ref/trigger/pipeline (or POST /projects/:id/ref/:ref/trigger/build for V3) endpoint.

Why was this MR needed?

To prevent returning 404 when the branch name contains a dot.

Does this MR meet the acceptance criteria?

• Changelog entry added
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29438 (closed)