Update Java Support to 17 minimum version

[rustyrazorblade]

Some workloads, such as CreateDrop, do a lot of allocation. Should improve that, but also, can use Shenandoah.

DEFAULT_JVM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC"

Should also bump the default heap size, and add instructions to the README explaining how to override the defaults with env vars.