Add vue3_migrate_project_overview feature flag

What does this MR do and why?

This is flag 1 of 7 for migrating the Create: Source Code Experience pages to Vue 3. Adds the vue3_migrate_project_overview beta feature flag (default off, actor-based) and identical vue3_migration.yml files (status: rollout) for the CE and EE pages/projects/show/ entrypoints.

How to validate

This covers all the apps listed in #613672. I did a manual spot check for the manual test cases listed with focus of the cases we don't have automation test coverage of.

References

• [Vue 3] Rollout vue3_migrate_project_overview (#613672)
• Epic: Migrate Create: Source Code Experience Apps to Vue 3
• Depends on: !250533 (merged)