Validate Issue#closed_at after #created_at

What does this MR do?

This adds a validation rule to Issue to ensure created_at < closed_at. This is necessary to isolate a bug where some Issue records appear to have been closed before they were opened. See Issue #33739 (closed) for more details.

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

This may slow import (mass creation of Issue records), but not by much.

First step on #33739 (closed)