Set mqtt-clientId to uptime-kuma_.. instead of mqttjs_.. (#4503)

server/monitor-types/mqtt.js

@@ -81,7 +81,8 @@ class MqttMonitorType extends MonitorType {
 
             let client = mqtt.connect(mqttUrl, {
                 username,
-                password
+                password,
+                clientId: "uptime-kuma_" + Math.random().toString(16).substr(2, 8)
             });
 
             client.on("connect", () => {