To support https://gitlab.com/groups/gitlab-org/-/work_items/21356+, the setting should be configurable from the REST API. The application and group setting updates are done in separate endpoints - the settings and groups APIs respectively. We should introduce a `use_built_in_project_templates_enabled` feature flag, and conditionally expose the setting based on the feature flag. An example MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/146582