Show milestone ID in a consistent way

What does this MR do?

Show milestone ID in a consistent way and format milestone header as issues and MRs

• Enclose the identifier in strong
• Use the % character as in GFM
• Remove spaces around en dash for date range

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)

What are the relevant issue numbers?

Closes #23602 (closed)