chore(deps): remove deprecated slack package & install @slack/bolt

Author: hivanalejandro

functions/billing/package.json

@@ -16,9 +16,9 @@
   "dependencies": {
     "@google-cloud/billing": "^4.0.0",
     "@google-cloud/compute": "^4.0.0",
+    "@slack/bolt": "^4.2.1",
     "google-auth-library": "^9.0.0",
-    "googleapis": "^143.0.0",
-    "slack": "^11.0.1"
+    "googleapis": "^143.0.0"
   },
   "devDependencies": {
     "@google-cloud/functions-framework": "^3.0.0",