Extend Vue 3 runner rollout to group and project pages

What does this MR do and why?

Extends the existing vue3_migrate_admin_runners rollout from the admin runner pages to the group and project runner pages. All of these entrypoints mount the same ~/ci/runner/* apps that already run under Vue 3 for admin, so they can share the flag rather than each waiting on a separate migration.

Two different mechanisms are used, per the migration guide:

Option 1 (vue3_migration.yml) — 9 standalone page entrypoints:

• pages/groups/runners/{index,show,edit,new,register}
• pages/projects/runners/{show,edit,new,register}

Option 2 (?vue3 partial migration) — the project runner list.

It has no standalone entrypoint: Projects::RunnersController#index redirects to the CI/CD settings page, where initProjectRunnersSettings() mounts alongside about a dozen unrelated apps. A vue3_migration.yml there would flip all of them, so only the runners app is moved, behind a gon.features check with a Sentry-reported fallback to the Vue 2 build.

No new feature flag is introduced — this reuses vue3_migrate_admin_runners.

How to verify

1. Enable the flag: Feature.enable(:vue3_migrate_admin_runners)
2. Visit any group or project runner page, plus Settings > CI/CD > Runners.
3. Confirm the console shows [V] Using Vue.js 3 (with @vue/compat) for ....
4. Confirm document.querySelectorAll('[data-gitlab-vue3-app]') returns the mounted runner app, and that on the CI/CD settings page only the runners app carries the marker.

The runners app on Settings > CI/CD, rendering under Vue 3:

Verified locally on that page with a temporary assertion in spec/features/projects/runners/maintainer_manages_project_runners_spec.rb:

• document.querySelectorAll('[data-gitlab-vue3-app]') returns exactly one element — ProjectRunnersSettingsAppRoot. None of the other apps on the settings page are affected, which is the point of using Option 2 here.
• The console logs [V] Using Vue.js 3 (with @vue/compat) for ProjectRunnersSettingsAppRoot.
• The full spec file passes (8 examples, 0 failures). Feature flags default to enabled in the test environment, so every example in it already exercises the page through the new ?vue3 path.
• spec/lib/gitlab/vue3_migration_files_spec.rb passes (162 examples), covering the 9 new YAML files.

References

• Feature issue: gitlab-org#20939
• Rollout issue: #594125