Upgrade postgres to v11

This is just for documentation, because the upgrade had to be done
in the UI

modules/gcloud/gcloud_postgres/main.tf

@@ -1,6 +1,6 @@
 resource "google_sql_database_instance" "db" {
   name             = var.database_instance_name
-  database_version = "POSTGRES_10_23"
+  database_version = "POSTGRES_11"
   region           = var.database_region
 
   lifecycle {