chore: Upgrade Firestore to v6.6.0

- Upgrade Firestore to v6.6.0 to support `FIRESTORE_PREFER_REST` environment variable

Author: lahirumaramba

package.json

@@ -206,7 +206,7 @@
     "uuid": "^9.0.0"
   },
   "optionalDependencies": {
-    "@google-cloud/firestore": "^6.5.0",
+    "@google-cloud/firestore": "^6.6.0",
     "@google-cloud/storage": "^6.9.5"
   },
   "devDependencies": {