Upgrade target Java verson for Atum server module

[salamonpavel]

It become increasingly problematic to use recent (if not any) dependencies given the old version of target Java version. As we need to stay up-to-date we need to upgrade target Java version for the server module. The target version should be 11+. The changes will involve build.sbt file and Dockerfile.

Once we compile to higher Java we will be able to bring in dependencies we need and improve test coverage (mainly endpoints).

[salamonpavel]

Done as part of #154