Source Code: Templates - Allow administrators to disable built-in project templates for project creation
## Related Issue
* https://gitlab.com/gitlab-org/gitlab/-/work_items/24112+
## Background
[Several customers](https://gitlab.com/gitlab-org/gitlab/-/work_items/24112) are experiencing confusion surrounding template selection and request the ability for administrators to toggle GitLab's built-in project templates through the Admin Area. The feature aims to help manage template clutter as the number of built-in and custom templates grows, ensuring only relevant templates appear in the "Create new project from template" flow.
This feature will be available to premium and ultimate customers.
## Goals
1. **Reduce confusion and errors:** Organizations with custom templates need users to find the right templates without built-in options adding noise
2. **Enforce governance and compliance:** Some organizations need to ensure only approved templates are available, particularly in regulated environments
3. **Align with governance expectations:** Organizations using server-side hooks or other controls expect all content entering repositories to pass through those checks. Built-in templates currently bypass this flow
### Scope
We will provide administrators the ability to disable built-in project templates with support for both:
* **UI**: Admin Area setting to enable/disable built-in templates globally
* **API**: Corresponding API endpoint to manage this setting programmatically
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