Fixes: #8950
The `IntegrationFlow.toReactivePublisher(true)` makes `PublisherIntegrationFlow`
to be stopped from the beginning and waiting for the `Publisher.subscribe()`.
However only `EndpointSpec` components are marked as `autoStartup(false)`.
The `MessageProducerSupport` is not included, therefore unexpected messages
are produced to the channel in the end of flow without subscribers.
* Check for `AbstractEndpoint` component type in the `PublisherIntegrationFlow`
and mark it as `setAutoStartup(false)`
* Ensure in a new `ReactiveStreamsTests.messageProducerIsNotStartedAutomatically()` test
that `MessageProducerSupport` is not started from the beginning,
but rather when we subscribe to the `Publisher` via `StepVerifier`
(cherry picked from commit 2b576c57d6)
30fa11f621