Resolve "Regressions for assigning group milestone from ancestor"

Fix the following regressions:

• System notes don't properly link to milestones when they belong to a parent group
• Quick-actions for assigning milestone is broken
• Side-bar has wrong link for milestone when it's edited, but fixes itself on refresh

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
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47735 (closed)