Showcase needs a refresh

I've noticed that several of the links are defunct in the showcase (e.g. tChat) or changed (e.g. [sbbscope](https://open-store.io/app/com.ubuntu.developer.psasse.sbbscope)). Plus, there is a nice and long and seemingly more up-to-date showcase at https://opentransportdata.swiss/en/showcase-2/ which we should link to.