chore(deps): bump firebase-admin in /functions from ^8.10.0 to ^12.0.0

firebase-admin v8 has reached end-of-life and has not received security
patches since 2021. v12 is the current stable series (requires Node 18+,
which aligns with Firebase Functions gen2). Multiple CVEs affect the
bundled grpc and google-auth-library transitive deps in v8 that are
resolved in v12.

Author: ZLeventer

functions/package.json

@@ -15,7 +15,7 @@
   "main": "lib/index.js",
   "dependencies": {
     "cors": "^2.8.5",
-    "firebase-admin": "^8.10.0",
+    "firebase-admin": "^12.0.0",
     "firebase-functions": "^3.13.2",
     "firebase-tools": "^9.10.0",
     "node-fetch": "^2.6.7"