[backport] SI-7237 Always choose ForkJoinTaskSupport

... on Java 6 and above.

ForkJoinTaskSupport works on Hotspot, Avian and J9,
while ThreadPoolTaskSupport causes the test
test/files/scalacheck/parallel-collections
to reliably hang on all three.

We keep ThreadPoolTaskSupport around to keep the hope
alive that we still have a glimpse of 1.5 support.