Add Vue 3 rollout flag for the admin deploy keys pages

Adds the vue3_migrate_admin_deploy_keys feature flag and a vue3_migration.yml for the admin deploy keys entrypoints. index opts into Option 1 (the bundler builds a Vue 2 and a Vue 3 chunk; Rails serves the Vue 3 chunk only when the flag is on; default-off). new mounts no Vue, so it goes straight to status: migrated with no flag.

Scope

• index — mounts DeployKeysTable (initVueApp, root DeployKeysTableRoot); status: rollout behind vue3_migrate_admin_deploy_keys.
• new — only initializes the Pikaday date picker (~/behaviors/date_picker), no Vue; status: migrated, no flag (same as ee/.../pages/projects/path_locks).

The :edit/:update routes have no page bundle, so there's nothing to migrate there.

Risk

Low. index is a single table component, no Apollo, and none of the known Vue 2→3 breakers ($on/$off/$once, Vue.observable, slot=, functional, filters). Covered by spec/frontend/admin/deploy_keys/components/table_spec.js. new mounts no Vue.

Verification

• bundle exec rspec spec/lib/gitlab/vue3_migration_files_spec.rb
• node scripts/frontend/vue3_migration_stats.mjs --owners

Local verification

References

• Migration issue: #628825
• Epic: &23285