Update dependency firebase-functions to v3.9.1

Author: renovate-bot

functions/package.json

@@ -14,7 +14,7 @@
   "main": "lib/index.js",
   "dependencies": {
     "firebase-admin": "8.12.1",
-    "firebase-functions": "3.6.1"
+    "firebase-functions": "3.9.1"
   },
   "devDependencies": {
     "typescript": "3.9.7",