Use HTTP 422 for changelog API errors

What does this MR do?

In !53890 (merged) it was brought up that an HTTP 500 error is a very generic error, and that an HTTP 422 error code is better suited for the changelog API.

This fixes #321676 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry Feature will be released in 13.9, and already had a changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content