Roll out Vue 3 for group page entrypoints

What does this MR do and why?

Rolls out Vue 3 (with @vue/compat) for group page entrypoints behind the vue3_migrate_group feature flag.

This MR adds vue3_migration.yml files with status: rollout to the following group page entrypoints:

• groups/activity
• groups/edit
• groups/group_members
• groups/new
• groups/show

When the feature flag is enabled for a user, Rails serves the Vue 3 chunk instead of the Vue 2 chunk for these pages.

References

• Migration issue: #608126
• Vue 3 migration epic: gitlab-org#23093
• Vue 3 migration guide

How to set up and validate locally

1. Follow the steps described in https://docs.gitlab.com/development/fe_guide/vue3_migration/#setup-gdk-to-use-vue-3-vuecompat
2. Enable the feature flag:

   Feature.enable(:vue3_migrate_group)

3. Navigate to any of the affected group pages (e.g., group overview, activity, settings, members, or new group)

Feature flag rollout

• Flag name: vue3_migrate_group
• Flag type: beta
• Actor: user

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #608126