Deprecate Vue 2

Following #1272

Vue 2 and 2.7 are not maintained anymore since 31st december 2023 (see https://v2.vuejs.org/eol/).

Dropping Vue 2 support is inevitable (from `features.js` and around), but in a new major version.
For the moment we can deprecate Vue 2/2.7 support and warn users about that (e.g.: a warning message when `.enableVueLoader()` is used with Vue 2).

Should be done before #1277 