Resolve "Mention @user dropdown does not suggest by non-ASCII characters in name"

What does this MR do?

• Support non-ASCII characters in GFM autocomplete

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

• Please closely review the modifications to the RegEx 😃

Why was this MR needed?

• Non-ASCII characters could be used in username, labels, etc but where not searchable with the GFM autocomplete

Screenshots (if relevant)

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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27067 (closed)