Drop GFM support for issuable title on milestone

What does this MR do?

Drop GFM support for issuable title on milestone, which was introduced in 2012 (GitLab 3.1) and has made it inconsistent.

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

No.

Why was this MR needed?

• To keep it consistent with other occurrences as GFM support is not introduced in any other occurrence of issuable title.
• To keep it faster using link_to instead of link_to_gfm.
  • e.g., a milestone with ~ 100 issues on my local laptop (GDK): 25x faster ✈ (2228 ms -> 88 ms)

Screenshots (if relevant)

as is

#4 is linked.

fixed

#4 is not linked but just treated as plan text.

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 #34289 (closed)