regionOrder property in OperationPreferences not synthesized to CloudFormation

[dannysteenman]

When using the OperationPreferences in cdk-stacksets, the regionOrder property is not being synthesized to the CloudFormation template. This prevents us from specifying the order in which regions should be updated during StackSet operations.

Reproduction Steps

1. Create a StackSet using the createStackSet function (or directly using the StackSet construct).
2. Specify operationPreferences including the regionOrder property.
3. Synthesize the CDK app.
4. Observe that the resulting CloudFormation template does not include the RegionOrder property in the OperationPreferences.

Expected Behavior

The synthesized CloudFormation template should include the RegionOrder property in the OperationPreferences section.