Resolve "Milestone Quick Action not displayed with no project milestones but with group milestones"

What does this MR do?

Allows the usage of /milestones quick action for group milestones

Steps to test

• Create a new group, GroupA

• Add a group milestone, Group Milestone

• Create a new project in GroupA, ProjectX

  • Do not create a project-level milestone

• Create a new issue in ProjectX

• Attempt to assign the new issue to the Group Milestone via quick action /milestone

• You should be able to assign the 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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42545 (closed)