Move labels entry points to Vue 3 rollout

What does this MR do and why?

This MR opts the Labels pages into the Vue 3 rollout. It changes no behaviour.

Each of the nine Labels entry points (group, project and admin, each with an index, new and edit page) now builds a Vue 2 chunk and a Vue 3 chunk. The Vue 3 chunk is served only when the new vue3_migrate_labels beta feature flag is on.

The flag is off by default.

References

• Feature work item: #608107
• Rollout issue: #618984
• Epic: gitlab-org#23066
• Pattern copied from: !249811 (merged)

Screenshots

Captured with vue3_migrate_labels enabled.

Group labels	Project labels	Edit label

How to set up and validate locally

1. Enable the flag in the Rails console: Feature.enable(:vue3_migrate_labels).
2. Restart Vite with gdk restart vite. Vite builds its page entry map at startup.
3. Visit the group Labels page at /:group/-/labels, the project Labels page at /:project/-/labels, and the admin Labels page at /admin/labels.
4. Open the New label and Edit label forms on each page.
5. Confirm each page renders and works as before.
6. Confirm the browser console shows Using Vue.js 3 (with @vue/compat) and no Vue errors.

MR acceptance checklist

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Related to #608107