Add count badges to both dashboard and group milestones

What does this MR do?

This MR adds count badges to both dashboard and group milestones, making them consistent with a project's milestones.

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

No.

Why was this MR needed?

To make the UI and UX consistent among different types of milestones.

Screenshots (if relevant)

Dashboard Milestones (before)

Dashboard Milestones (after)

Group Milestones (before)

Group Milestones (after)

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 #24421 (closed)

This is my first MR, so please let me know if there is a better alternative to implement these changes or if / how I can improve this MR (maybe add more tests?). Thanks a lot.