Revert "Refactor AuditEvent to use target_type database value"

What does this MR do?

This reverts !36198 (merged), so that we can create it again with the addition of target_type to be added in a regular migration.

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