Missing java-vm-args to use ShenandoahGC

[buedi]

In the JNLP file I set the Shenandoah GC with the vm-args:

but I got the warning:

[ITW-CORE][WARN ][net.sourceforge.jnlp.Parser] Ignoring java-vm-args due to illegal Property -XX:+UseShenandoahGC