Migrate project page entrypoints to Vue 3

What does this MR do and why?

Rolls out Vue 3 (with @vue/compat) for project page entrypoints behind a new vue3_migrate_project feature flag, following the existing vue3_migration.yml-based mechanism (Option 1 in the Vue 3 migration guide).

This MR adds vue3_migration.yml files with status: rollout and feature_flag: vue3_migrate_project to the following entrypoints:

• projects/activity
• projects/edit (and its EE equivalent)
• projects/new (and its EE equivalent)
• projects/project_members

projects/show (and its EE equivalent) was intentionally left out of this flag's scope and will be handled in a separate issue.

References

• Migration issue: #608127
• 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_project)

3. Navigate to any of the affected project pages (activity, new, edit, or members) and verify the console shows [gitlab] [V] Using Vue.js 3 (with @vue/compat) for the mounted apps

Feature flag rollout

• Flag name: vue3_migrate_project
• 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 #608127