Provide current and LTS showcases for React and Angular

Especially to migrate from Angular 13, but to provide version Angular 14 and 15 showcases we'll introduce `current` and `lts` showcase folders at least for Angular and React, as [Vue](https://github.com/vuejs/roadmap/issues/9#issuecomment-380525737) doesn't seem have a lower major LTS version (any more) at the moment.