Include milestones from subgroups in Milestones view

What does this MR do?

#24779 (closed)

When visiting the milestones list at a group level, milestones that belong to subgroups and subgroup projects should be displayed as long as the user have access to them

Screenshots

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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

Closes #24779 (closed)