Drop GFM support for the title of Milestone/MergeReuqest in template

What does this MR do?

Drop GFM support for the title of Milestone/MergeReuqest in templates.

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

No.

Why was this MR needed?

The title in Milestone/MergeReuqest is expected as plain, not as markdown, like in other occurrences. This MR keeps it consistent with them.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests
  • [n/a] 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 #34309 (closed)