Refuse to perform an LFS clean on projects that are fork roots

What does this MR do?

When a project is at the root of a fork network, an intersection of technical debt means that all child forks have a claim on the LFS objects owned by the fork parent. This makes it dangerous to remove any LFS references in the fork root without checking all the repos for all the children, which quickly mounts up so as to be unmanageable.

For now, just skip projects like this.

If a fork is an internal or leaf node in the fork graph, they may have some LFS objects of their own. Those references are safe to prune.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Closes #244848 (closed)