Hide subgroup milestones on Roadmap when filtered by group

What does this MR do and why?

Previously, when selecting Show group milestones in Roadmap settings, it would include subgroup milestones. This change filters out subgroup milestones to only show current group milestones.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Have at least one group which contains one subgroup and one project.
2. At group level, create one milestone (Issues > Milestones > New Milestone) with a start date and due date.
3. At sub-group level, create one milestone (Issues > Milestones > New Milestone) with a start date and due date.
4. At project level, create one milestone (Issues > Milestones > New Milestone) with a start date and due date. You should now have at least 3 milestones (one at every hierarchy level).
5. At group level, create at least an epic with start and due dates to display data in the Roadmap view.
6. At group level, go to Epics > Roadmap > click on Settings to open the Roadmap settings in the right panel.
7. In the Roadmap settings, use the filter Show group milestones, it now returns only group and sub-group milestones.

MR acceptance checklist

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Related to #359948 (closed)