fix(examples): Use another public broker for MQTT example

Author: david-cermak

examples/mqtt/main/Kconfig.projbuild

@@ -2,7 +2,7 @@ menu "Example Configuration"
 
     config BROKER_URL
         string "Broker URL"
-        default "mqtt://mqtt.eclipseprojects.io"
+        default "mqtt://test.mosquitto.org"
         help
             URL of the broker to connect to