Retire vue3_migrate_repository feature flag

What does this MR do and why?

Retires the vue3_migrate_repository feature flag, which has been default_enabled: true since 19.1. The tree and blob page entrypoints it gated are fully migrated to Vue 3 and the flag no longer provides any rollout value. This is Phase 0 of the Migrate Create: Source Code Experience Apps to Vue 3 epic.

Screenshots or screen recordings

Page	Recording
Project overview
Tree page
Blob page

How to set up and validate locally

1. Check out this branch and run bundle install / yarn install if prompted.
2. Restart Vite and Rails in GDK (gdk restart vite rails).
3. Open a project in the browser and visit the project overview (/group/project), a tree page (/group/project/-/tree/main), and a blob page (/group/project/-/blob/main/README.md).
4. Open the browser console and run the following snippet to confirm Vue 3 app roots are present on each page:

[...document.querySelectorAll('[data-v-app]')].map((el) => el.__vue_app__?._component?.name)

Expect the array to contain component names (not undefined or empty), confirming Vue 3 instances are mounted.

5. Open the Network tab, filter by JS, and confirm the treeList and blobViewer chunks load only their Vue 3 variants (no plain Vue 2 treeList/blobViewer chunk requests).

References

• Resolves #613671 (closed)
• Epic: &23084

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