Change HTTP Status Code when repository disabled

What does this MR do?

This merge request closes #50834 (closed).

Instead of going with the 204 as suggested in the issue, I felt like 404 was more appropriate (especially so for the POST and DELETE requests).

I do not believe there are any additional risks, as there is still authorization taking place after the check.

Bear with me, this is my first Merge Request 👍

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team