Update dependency mqtt to v2.18.8

Author: renovate[bot]

iot/beta-features/commands/receive/package.json

@@ -13,7 +13,7 @@
   "dependencies": {
     "@google-cloud/pubsub": "0.13.2",
     "jsonwebtoken": "^8.3.0",
-    "mqtt": "2.15.0",
+    "mqtt": "2.18.8",
     "yargs": "8.0.2"
   },
   "devDependencies": {

iot/mqtt_example/package.json

@@ -10,7 +10,7 @@
   },
   "dependencies": {
     "jsonwebtoken": "8.4.0",
-    "mqtt": "2.16.0",
+    "mqtt": "2.18.8",
     "yargs": "11.0.0"
   },
   "devDependencies": {