Fix broken specs depending on UnauthorizedError

What does this MR do?

Gitlab::GitAccess::UnauthorizedError was just renamed to ForbiddenError. An MR merged shortly after referenced ForbiddenError. This simple rename should be sufficient to get the pipeline green again.

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

Closes #208290 (closed)