Important notes if your parent domain zone is not under the same project:
You need to set auto_create_ns_record = false to disable subdomain NS record auto creation in the parent domain.
Perform terraform apply.
After terraform apply is finished, copy the name servers value from terraform output and manually add them to parent domain as NS record. If you are not using Cloud DNS as the parent domain, please refer to your Domain Registrar documentation on how to add NS record.