Doc: Add valid value to QMqttClient::protocolVersion

As described in the ProtocolVersion enum description, MQTT Standard 5
is also a valid value.

Fixes: QTBUG-120947
Pick-to: 6.7 6.5 6.2 5.15
Change-Id: I27ed8ad1d31201abb195c35ab56ced3ad08494cc
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

src/mqtt/qmqttclient.cpp

@@ -82,6 +82,7 @@ Q_LOGGING_CATEGORY(lcMqttClient, "qt.mqtt.client")
     \list
         \li 3: MQTT standard version 3.1.
         \li 4: MQTT standard version 3.1.1, often referred to MQTT 4.
+        \li 5: MQTT Standard 5.0.
     \endlist
 */