Enable Vue 3 migration rollout for group roadmap page

What does this MR do and why?

Enables the Vue 3 migration rollout for the group roadmap page (ee/app/assets/javascripts/pages/groups/roadmap/show). This uses the new vue3_migration.yml mechanism from !234347 (merged) to opt the page into Vue 3 behind a feature flag. When vue3_migrate_roadmap is enabled, Rails serves the Vue 3 chunk; otherwise the page continues to run on Vue 2.

What changed

• Added config/feature_flags/beta/vue3_migrate_roadmap.yml
• Added ee/app/assets/javascripts/pages/groups/roadmap/show/vue3_migration.yml with status: rollout

No JavaScript or controller changes are needed.

How to verify

1. Enable the feature flag: Feature.enable(:vue3_migrate_roadmap)
2. Navigate to any group's roadmap page
3. Open the browser console and confirm the message: [gitlab] [V] Using Vue.js 3 (with @vue/compat) for RoadmapRoot
4. Verify the roadmap renders and functions correctly