Update dependency @google-cloud/firestore to v3 (#1567)

Author: renovate-bot

functions/firebase/package.json

@@ -23,6 +23,6 @@
     "uuid": "^3.1.0"
   },
   "dependencies": {
-    "@google-cloud/firestore": "^2.0.0"
+    "@google-cloud/firestore": "^3.0.0"
   }
 }

functions/node8/package.json

@@ -25,7 +25,7 @@
     "uuid": "^3.3.2"
   },
   "dependencies": {
-    "@google-cloud/firestore": "^2.0.0",
+    "@google-cloud/firestore": "^3.0.0",
     "escape-html": "^1.0.3",
     "node-fetch": "^2.3.0"
   }