Use Java 21 for half of docker-latest builds

Hi devs,

Our java support strategy says that we should move to support java 21 ASAP. Right now we use it in docker-unsupported and I propose that we move half of of our builds of docker-latest to java 21 (and keep half of the rest on Java 17).



PS: I tried checking our last docker-unsupported results but it seems the build stopped quickly because of an OOM. There were 35 tests which passed though so at least there’s no major problem running on java 21.