Release Note: JEP 376: ZGC Concurrent Stack Processing

The Z Garbage Collector now processes thread stacks concurrently. This allows all roots in the JVM to be processed by ZGC in a concurrent phase instead of stop-the-world pauses. The amount of work done in ZGC pauses has now become constant and typically not exceeding a few hundred microseconds.

For further details, see [JEP 376](https://openjdk.java.net/jeps/376).