Update JupyterHub to 4.*

Update JupyterHub using a new Helm chart version 3.* (currently we use Helm chart version 2.0.0, which results in JupyterHub 3.0.0). The update procedure is described here. For JupyterHub version 4 the data base scheme needs to be updated, see changelog of Helm chart and changelog of JupyterHub.