Add Vue 3 rollout flag for boards pages

What does this MR do and why?

This MR adds the vue3_migrate_boards feature flag and a vue3_migration.yml for three boards page entrypoints. This opts them into the Vue 3 migration with Option 1.

Entrypoints

• pages.projects.boards
• pages.groups.epic_boards
• pages.groups.boards

References

https://docs.gitlab.com/development/fe_guide/vue3_migration/#option-1-recommended-migrate-your-page-entrypoint-using-a-feature-flag-and-vue3_migrationyml

Screenshots or screen recordings

There are three routes to verify:

• Group boards (eg. http://gdk.test:3000/groups/twitter/-/boards/5)

Console:

• Project boards (eg. http://gdk.test:3000/twitter/Typeahead.Js/-/boards)

Console:

• Epic boards (eg. http://gdk.test:3000/groups/twitter/-/epic_boards)

Console:

How to set up and validate locally

1. Enable the feature flag vue3_migrate_boards
2. Confirm the board loads in vue 3 mode for all three pages above by (The browser console should show something like Using Vue.js 3 for BoardAppRoot)
3. Confirm the browser console stays free of any Vue 3 specific errors
4. Please pay special attention to card reording, moving, dragging, and dropping. This needs explicit manual QA

MR acceptance checklist

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 #608115 (closed)