Mention in docs that org.eclipse.paho.client.mqttv3 is optional

Author: artembilan

src/reference/antora/modules/ROOT/pages/mqtt.adoc

@@ -27,6 +27,7 @@ compile "org.springframework.integration:spring-integration-mqtt:{project-versio
 ======
 
 The current implementation uses the https://www.eclipse.org/paho/[Eclipse Paho MQTT Client] library.
+Starting with version 6.5, the `org.eclipse.paho:org.eclipse.paho.client.mqttv3` dependency is an `optional` dependency, so has to be included explicitly in the target project for MQTT v3 support.
 
 IMPORTANT: The XML configuration and most of this chapter are about MQTT v3.1 protocol support and respective Paho Client.
 See xref:mqtt.adoc#mqtt-v5[MQTT v5 Support] paragraph for respective protocol support.

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -44,4 +44,10 @@ See xref:message-store.adoc#use-lock-registry[Use LockRegistry] for more informa
 
 The `SourcePollingChannelAdapter` endpoint now starts a `CONSUMER` kind observation for the received message.
 The `MessageReceiverContext` now distinguishes between `handler`, `message-source` and `message-producer` values for the `spring.integration.type` low cardinality tag.
-See xref:metrics.adoc#micrometer-observation[Micrometer Observation] for more information.
+See xref:metrics.adoc#micrometer-observation[Micrometer Observation] for more information.
+
+[[x6.4-mqtt-changes]]
+== Optional Paho MQTT Dependencies
+
+The `org.eclipse.paho:org.eclipse.paho.client.mqttv3` dependency for `spring-integration-mqtt` is now also optional as `org.eclipse.paho:org.eclipse.paho.mqttv5.client` always was.
+See xref:mqtt.adoc[MQTT Support] for more information.