Revise Javadoc on AsyncHandlerMethodReturnValueHandler to clarify its
main purpose is to prioritze custom async return value handlers ahead
of built-in ones. Also replace the interface from built-in handlers
which are prioritized already.
Remove DeferredResultAdapter and ResponseBodyEmitterAdapter --
introduced in 4.3 for custom async return value handling, since for
5.0 we will add built-in support for reactive types and the value of
these contracts becomes very marginal.
Issue: SPR-15365
ae1ed16cb8