[5.x] Upgrade from Vue 2 to Vue 3

[osbre]

Vue 2 has reached EOL. I think it's a good idea to migrate to Vue 3 since there are only a few changes to make it happen:

1. Update dependencies and Vue initialization in app.js.
2. Remove redundant/unused addTagModalClosed event dispatching due to the event bus being deprecated and the event itself being unused by Horizon.
3. Reorganize the routes as nested so they still highlight as active in the sidebar due to the change in vue-router v4 active link logic.

I have tested the app and it works perfectly fine as before. Vue 3 💚

[github-actions]

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