Add path of project and group milestones

What does this MR do?

Adds the project and group name to the return type for project and group milestones.

Why was this MR needed?

To display the project and group paths for a given milestone.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to 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
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#49292 (closed) - View path of group milestone on dashboard milestone page