This commit defers flushing of the response until the first item is
emitted that needs to be written (and flushed) to the response.

This makes Spring MVC consistent with WebFlux in this regard.

Closes gh-21972

Closes gh-22035

ServerRequest:
 - awaitPrincipalOrNull is renamed to awaitPrincipal since
 there is no non-nullable variant

ServerResponse:
 - new BodyBuilder.sse() extension
 - BodyBuilder.bodyToServerSentEvents is deprecated in favor
   of sse().body()
 - BodyBuilder.bodyAndAwait(flow: Flow<T>) is renamed to
   bodyFlowAndAwait to avoid shadowing of
   BodyBuilder.bodyAndAwait(body: Any)
 - BodyBuilder.bodyToServerSentEventsAndAwait is removed,
   sse().bodyAndAwait() should be used instead

Closes gh-22899

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

See gh-22894

* pr/22891:
  Polish contribution
  Polish CONTRIBUTING.md

Closes gh-22891

See gh-22891

Includes combined imports of assertion methods and related polishing.

Closes #22889

Closes gh-22888

Aligned with invocation.reactive and transaction.reactive packages.

Closes gh-22887

Closes gh-22837

Closes gh-22870

We now use Flux.usingWhen() instead materialize/dematerialize operators
to reuse Reactor's resource closure.

Until usingWhen() accepts a BiFunction to consume error signals,
we need to map error signals outside of usingWhen which requires
re-wrapping of the ReactiveTransaction object.

doCleanupAfterCompletion gets now subscribed to when processing
cleanup after a new transaction.

Closes gh-22877

Closes gh-22869

Such a static unmodifiable TransactionDefinition with defaults can be used for getTransaction(null) calls, now also possible for getReactiveTransaction. Furthermore, it can be used for a simple TransactionalOperator.create(ReactiveTransactionManager) method without an internal dependency on the transaction.support package.

See gh-22646

Closes gh-22874