To support https://gitlab.com/groups/gitlab-org/-/work_items/21356+, we want to add a cascading setting that can manage the feature on an instance and group level. Following the [cascading settings framework](https://docs.gitlab.com/development/cascading_settings/), we need to add the new `built_in_project_templates_enabled` attribute as a `cascading_attr`. An example MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/189272