* NodeJS: v16
* NPM: v8
* Jest: v27
* Mongoose: v6

And fix some small issues along the way

* introduce a test on the server to see if user creation works (backend process <-> db only;
  still needs a test that involves the router, too)
* main server process is no connection-aware and wont start if database is not reachable
* some renaming, cause it's more descriptive
* unify quote usage

src: https://github.com/aamirpinger/todo-app-client-server-kubernetes/commit/93be212a344ea910b0a13f1110975bfdfd306f3a