ThreadContainer registry accumulates weak refs

This is memory leak / regression since JDK 19 due to the addition of the registry of thread containers. If you create lots of thread pools, don’t close them, let them be GC’ed, then the weak refs to these contains aren't expunged from the registry. See analysis from Alexey Gavrilov here:

https://mail.openjdk.org/pipermail/loom-dev/2023-May/005613.html