Fix MigrateApproverToApprovalRules migration

What does this MR do?

At this stage of migration, the rule_type column is not yet present, therefore in migration the older column must be referenced, meaning having the older logic copied into Gitlab::BackgroundMigration::MigrateApproverToApprovalRules

See #11927 (closed) for failure

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.