fix: Allow managing allocated_storage for replicas

A regression was introduced via c66cd73 where allocated_storage can only be
set on primary instances. However, this should not be a limitation because RDS
does allow managing replica storage independently from the primary.

modules/db_instance/main.tf

@@ -35,7 +35,7 @@ resource "aws_db_instance" "this" {
   engine            = local.is_replica ? null : var.engine
   engine_version    = var.engine_version
   instance_class    = var.instance_class
-  allocated_storage = local.is_replica ? null : var.allocated_storage
+  allocated_storage = var.allocated_storage
   storage_type      = var.storage_type
   storage_encrypted = var.storage_encrypted
   kms_key_id        = var.kms_key_id