Don't use MilestoneFinder for group milestones

What does this MR do?

This refactors the DashboardGroupMilestone model so that it no longer uses the MilestoneFinder, instead using methods defined on the Milestone model directly. According to our guidelines for reusing abstractions model class methods should not reuse finders.

What are the relevant issue numbers?

Closes #51546 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides