Explore usage of virtual threads for Observables

[andreacalia]

Virtual threads can have significant benefits on the Observables implementation.
Since every operation is dispatched to the pool, we can use quite a lot of platform threads unnecessarily.

Change in the code should be quite straightforward, but we would raise minimum JDK 21

ossgang-commons/src/main/java/org/ossgang/commons/observables/DispatchingObservable.java

Line 54 in 63a6f11

private static final ExecutorService DISPATCHER_POOL = newCachedThreadPool(