Use handle/flatMapIterable instead of flatMap/flatMapMany
when possible.

Closes gh-22727

See gh-21855

Transform the beanName in method DefaultListableBeanFactory.isPrimary.
Add the corresponding test cases.
Fixes #22675.

Closes gh-2080

Closes gh-22546

Closes gh-22605

Closes gh-22708

Closes gh-22694

This reverts commit 18f2e6a1

See gh-22720

1. If the write Subscriber cancels with the item cached, release it.

2. If the write Publisher emits an error while the item is cached, when
the write Subscriber subscribes, release the cached item and emit the
error signal.

Closes gh-22720

Closes gh-22719

This commit improves CORS support by:
 - Using CORS processing only for CORS-enabled endpoints
 - Skipping CORS processing for same-origin requests
 - Adding Vary headers for non-CORS requests

It introduces an AbstractHandlerMapping#hasCorsConfigurationSource
method in order to be able to check CORS endpoints efficiently.

Closes gh-22273
Closes gh-22496

Closes gh-22717

* pr/22718:
  Polish contribution
  Polish

Closes gh-22718

See gh-22718

As of Coroutines 1.2.0-alpha, Dispatchers.Unconfined
is a stable API so we can leverage it in order to get
better performances in our Reactive to Coroutines
bridge.

See gh-19975

This commit also switches on kotlin-compiler-embeddable to
improve isolation of its dependencies

See gh-19975

* pr/22709:
  Polish contribution
  Polish

Closes gh-22709

See gh-22709

Closes gh-21670

The cancellation callback in asynchronousReadFileChannel must be called
before the first read I/O or otherwise if cancellation signals happens
immediately the onDispose callback may be missed.

The DefaultBufferFactory workaround however remains in place until an
expected additional fix arrives with Reactor Core 3.2.9.

See gh-22107