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

## 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. 

Closes #22452

See merge request !6533