Roadmap settings - Include subgroup in group milestones

What does this MR do and why?

Follow up from !80248 (merged)

Includes subgroup milestones when selecting group milestones from sidebar settings.

Context: #352331 (comment 841838461)

Behind :roadmap_settings feature flag.

Screenshots or screen recordings

How to set up and validate locally

• Turn on feature flag :roadmap_settings
• Visit a non empty roadmap
• Click on Settings in the top right corner
• A drawer with the Roadmap settings title shows up
• Test Milestones radio buttons and toggle
• URL gets updated when changing options and milestones are updated.
• Subgroup milestones are displayed when group milestones is selected.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352331 (closed)