Fix permissions for group milestones

What does this MR do?

Group milestones permissions were being delegate to Project which is nil. This was causing :read_milestone check return false for group milestones.

This causing the problem described on #32364 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers