Fixes: https://github.com/spring-projects/spring-integration/issues/9538
Some applications might not be satisfied with `.publishOn(Schedulers.boundedElastic())`
used by default for `Mono` replies.
* Remove that `.publishOn(Schedulers.boundedElastic())` from the `AbstractMessageProducingHandler`.
Instead, the target project is free to make a choice via `customizeMonoReply()`, e.g.:
```
.handle(RSockets.outboundGateway("/lowercase")
.clientRSocketConnector(this.clientRSocketConnector),
endpoint -> endpoint.customizeMonoReply((message, mono) ->
mono.publishOn(Schedulers.boundedElastic())))
```