Add group milestones to dashboard milestone page

What does this MR do?

Adds the ability to view group milestones on the dashboard milestone page.

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

Why was this MR needed?

In order to view group milestones on the dashboard milestone page.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #35748 (closed) - View group milestones on dashboard milestone page