fix: Addresses persistent diff with manage_default_network_acl

As noted in the [terraform docs][0], subnets using the default network acl
will generate a persistent diff if they are not specified to the aws_default_network_acl
resource. This module was handling subnets created by the module, but
of course is not aware of subnets created externally to the module.

The docs suggest using lifecycle ignore_changes as an option to avoid
the persistence diff, which is the approach implemented in this patch.

Fixes terraform-aws-modules#529

[0]: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/default_network_acl#managing-subnets-in-a-default-network-acl

main.tf

@@ -587,28 +587,9 @@ resource "aws_default_network_acl" "this" {
 
   default_network_acl_id = aws_vpc.this[0].default_network_acl_id
 
-  # The value of subnet_ids should be any subnet IDs that are not set as subnet_ids
-  #   for any of the non-default network ACLs
-  subnet_ids = setsubtract(
-    compact(flatten([
-      aws_subnet.public[*].id,
-      aws_subnet.private[*].id,
-      aws_subnet.intra[*].id,
-      aws_subnet.database[*].id,
-      aws_subnet.redshift[*].id,
-      aws_subnet.elasticache[*].id,
-      aws_subnet.outpost[*].id,
-    ])),
-    compact(flatten([
-      aws_network_acl.public[*].subnet_ids,
-      aws_network_acl.private[*].subnet_ids,
-      aws_network_acl.intra[*].subnet_ids,
-      aws_network_acl.database[*].subnet_ids,
-      aws_network_acl.redshift[*].subnet_ids,
-      aws_network_acl.elasticache[*].subnet_ids,
-      aws_network_acl.outpost[*].subnet_ids,
-    ]))
-  )
+  # subnet_ids is using lifecycle ignore_changes, so it is not necessary to list
+  # any explicitly. See https://github.com/terraform-aws-modules/terraform-aws-vpc/issues/736.
+  subnet_ids = null
 
   dynamic "ingress" {
     for_each = var.default_network_acl_ingress
@@ -644,6 +625,10 @@ resource "aws_default_network_acl" "this" {
     var.tags,
     var.default_network_acl_tags,
   )
+
+  lifecycle {
+    ignore_changes = [subnet_ids]
+  }
 }
 
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