chore(deps): bump mqtt from 4.2.1 to 4.2.6

Bumps [mqtt](https://github.com/mqttjs/MQTT.js) from 4.2.1 to 4.2.6.
- [Release notes](https://github.com/mqttjs/MQTT.js/releases)
- [Commits](mqttjs/MQTT.js@v4.2.1...v4.2.6)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -105,7 +105,7 @@
     "express": "^4.17.1",
     "jsonfile": "^6.0.1",
     "morgan": "~1.10.0",
-    "mqtt": "^4.2.1",
+    "mqtt": "^4.2.6",
     "mqtt-nedb-store": "^0.1.1",
     "native-url": "^0.3.4",
     "nedb": "^1.8.0",