feat(infra): allow Cloud Functions to scale to 0

this saves us money

Refs: HOLI-10945

terraform/environments/event_listener.tf

@@ -50,7 +50,7 @@ resource "google_cloudfunctions2_function" "event_listener_function" {
     all_traffic_on_latest_revision = true
     ingress_settings               = "ALLOW_INTERNAL_ONLY"
 
-    min_instance_count = 1
+    min_instance_count = 0
     max_instance_count = 1
   }