Upgrade to Gradle 7 and Java 11

This MR moves this app to Java 11 and Gradle 7 which will need to happen eventually. It took me some time to get there and to not freak out I needed to take some shortcuts, most of which have TODOs in the code. Although CI is passing again and local builds also work for me, there is some work left to complete before this should be merged. This work is ideally done by someone who is not me.

Related to !980 (closed) Supersedes !983 (closed)