fix(deps): update dependency @google-cloud/billing to v4 (#3398)

Co-authored-by: Patti Shin <[email protected]>

Author: renovate-bot

functions/billing/package.json

@@ -14,7 +14,7 @@
   "author": "Ace Nassri <[email protected]>",
   "license": "Apache-2.0",
   "dependencies": {
-    "@google-cloud/billing": "^3.0.0",
+    "@google-cloud/billing": "^4.0.0",
     "@google-cloud/compute": "^3.1.0",
     "google-auth-library": "^9.0.0",
     "googleapis": "^123.0.0",