gradle builds: define Java language version

[MoritzKeppler]

We should set a fixed java language version. Best would be to stick to latest LTS, i.e. java11.
Applies for gradle builds and docker file creation.
_{Moritz Keppler moritz.keppler@daimler.com, Daimler TSS GmbH, legal info/Impressum}

[DominikPinsel]

Another note from me:
I just saw that the java version for the external:nifi:processors project needs to be java8.

If I compile it with java11 and copy it to my local nifi docker image I get the following error:
Failure to launch NiFi due to java.lang.UnsupportedClassVersionError: microsoft/dagx/transfer/nifi/processors/FetchS3Object has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0

[jimmarino]

Yeah, unfortunately that seems to be one of the strange limitations with NiFi at the moment.

[paullatzelsperger]

that's why in the CI build we use Java 8 as base image for the custom Nifi image. Check .github/workflows/build-docker.yaml, Line 36ff