Update `AvailabilityChangeEvent` to be a `PayloadEvent` and ensure
that the `getResolvableType` method returns a generic compatible
result.
Prior to this commit, a ClassCastExeption would be thrown if the
following event listener was declared:
@EventListener
void onEvent(AvailabilityChangeEvent<ReadinessState> event) {
...
}
Closes gh-21898
1604545727