Dataflow runner - be more lenient with agent java version

DataflowRunner currently uses Environments.getJavaVersion to detect java version and build the Dataflow user agent string.

Environments.getJavaVersion only accept a small subset of java version (8, 11 and 17)

It would be nice the user agent string to accept more exotic java versions to allow running the agent on more exotic jvm (still keeping the beam model itself in java 11).