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

And fix some small issues along the way

* fix file name typo
* remove logs in test do reduce noise
* prevent 'connection established' log in test mode to reduce noise
* rename test to make it more obvious that it does not require (or create)
  a user object

* update mongoose, jest and nodemon to address security alerts triggered
  by dependabot
* 'test' target had a bug introduced by using wrong syntax
* renamed server scripts to clarify the distinction between invocations
  in local development, CI and actual operations

* adjust mongoose configuration
* documents more make targets in readme
* applied npm scripts convention of the client to the corresponding
  server scripts in order to be more consistent (`start` is now meant
  for local development)

* change behaviour so that service still comes up regardless whether db connection
  can be established or not (see 12-Factor-App) - loosely tight to backing service
* instead, it will log to stderr hinting this issue
* successful port binding is still required

* 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