Previously, an AvailabilityChangeEvent was published when the servlet
and reactive web server application contexts were closed, irrespective
of whether or not the context was active. This caused problems when
the context was not active due to a refresh failure as the event
publication could then trigger bean creation and post-processing that
relied upon beans that had been destroyed when cleaning up after the
refresh failure. The most commonly seen symptom was a missing
importRegistry bean that is required by ImportAwareBeanPostProcessor.
This commit updates the two web server application contexts to only
publish the availability change event if the context is active.
Fixes gh-21588
f17f1255a4