doc: mqtt: Add mention of MQTT 5.0 support

MQTT 5.0 is now supported so reflect this in the library documentation.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

doc/connectivity/networking/api/mqtt.rst

@@ -18,7 +18,7 @@ For more information about the protocol itself, see http://mqtt.org/.
 Zephyr provides an MQTT client library built on top of BSD sockets API. The
 library can be enabled with :kconfig:option:`CONFIG_MQTT_LIB` Kconfig option and
 is configurable at a per-client basis, with support for MQTT versions
-3.1.0 and 3.1.1. The Zephyr MQTT implementation can be used with either plain
+3.1.0, 3.1.1 and 5.0. The Zephyr MQTT implementation can be used with either plain
 sockets communicating over TCP, or with secure sockets communicating over
 TLS. See :ref:`bsd_sockets_interface` for more information about Zephyr sockets.