* Remove usage of non-stable `org.springframework.util.SocketUtils`
* Replace it with `0` for those tests where it is possible to select OS port
* Remove the mentioning of the `SocketUtils` from the `testing.adoc`
* Use `TransportConstants.DEFAULT_STOMP_PORT` for `StompServerIntegrationTests`.
We may disable this test in the future for CI if it is not going to be stable
* Introduce `Supplier<String> connectUrl` variants for `ZeroMqMessageHandler`
to let it defer connection evaluation until subscription to the socket `Mono`
in the `ZeroMqMessageHandler`.
* Move connection logic in the `ZeroMqMessageHandler` to `Lifecycle.start()`
Related to https://github.com/spring-projects/spring-framework/issues/28054
**Cherry-pick to `5.4.x`**
c661d7925e