Show releases on group milestones

What does this MR do and why?

Shows releases on group milestones; currently shown on the milestones list but not in the milestone sidebar.

Removes project-only limitation since group milestones can be linked to project releases.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
		Group milestone (1)
		Group milestone (4+)
		Project milestone — no change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a group-level milestone in a licensed group
2. Create release(s) in a project within that group, and link to the group milestone
3. Refresh the group milestone page