update to follow change in aws_db_instance id

aws_db_instance was updated by hashicorp/terraform-provider-aws#31232
such that the `id` attribute now tracks the resource ID, and the
identifier is now returned in a new attribute named `identifier`.

The resource ID is already available in `instance_resource_id`. This
fix allows existing users to continue using this module with no
changes and preserves the original function of the `id` attribute
in this module.

Fixes terraform-aws-modules#495.

modules/db_instance/outputs.tf

@@ -50,7 +50,7 @@ output "db_instance_hosted_zone_id" {
 
 output "db_instance_id" {
   description = "The RDS instance ID"
-  value       = try(aws_db_instance.this[0].id, "")
+  value       = try(aws_db_instance.this[0].identifier, "")
 }
 
 output "db_instance_resource_id" {