Resolve #22452"Unnecessary milestone name's characters html escaping"

What does this MR do?

This MR fixes a bug that unnecessary escapes reserved HTML characters for Milestone's title. See #22452 (closed).

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

• Unescaping of sanitized milestone title before it is being stored in the database. See Milestone#title and a private method called Milestone#sanitize_title
• Sufficient tests were added (Model and API tests were modified/added).

Why was this MR needed?

To allow reserved HTML characters in a milestone's title, such as "PHP migration 5.6 -> 7.0". The text appears in 'milestones' and in a dropdown during issue creation, issue list, and in another dropdown for issue filter.

Screenshots

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22452 (closed)