Add burndown chart on group milestone page in Starter

What does this MR do?

Adds a burndown chart on the group milestone page in Starter.

Why was this MR needed?

So that Starter can view burndown charts on their group milestone page.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6251 (closed) - Burndown chart on group milestone page in Starter.