Bumps everything to OpenJDK 15.
Docker image switched to Azul OpenJDK, because they already have 15 and also because they don't require glibc on Alpine, possibly leading to even smaller images.
Javadoc no longer fails on warnings, as 15 has some unwelcome changes there.
Finally, CI now caches Maven artifacts for build speedup.
Bumps everything to OpenJDK 15. Docker image switched to Azul OpenJDK, because they already have 15 and also because they don't require glibc on Alpine, possibly leading to even smaller images. Javadoc no longer fails on warnings, as 15 has some unwelcome changes there. Finally, CI now caches Maven artifacts for build speedup.