[AWS::RDS::DBCluster] - [Ignore PreferredBackupWindow or BackupRetentionPeriod if AWS Backup is managing PITR backups] -

### Name of the resource

AWS::RDS::DBCluster

### Resource name

_No response_

### Description

If a customer has a CFN stack with AWS::RDS::DBCluster or AWS::RDS::DBInstance that specifies PreferredBackupWindow or BackupRetentionPeriod, and the customer then turns on AWS Backup to manage PITR backups for the cluster/instance, the CFN stack will be marked as in drift.

The customer asks the CFN resource to ignore PreferredBackupWindow or BackupRetentionPeriod if AWS Backup is managing PITR backups for the resource (i.e. CFN should not give errors about drift).

The customer also reports that currently they can work around this somewhat by removing PreferredBackupWindow from existing stacks-- but they still need to maintain BackupRetentionPeriod in existing stacks to exactly match the current AWS Backup setting, which is tedious for them.

### Other Details

for AWS staff... internal PFR link: https://aws-crm.lightning.force.com/lightning/r/Product_Feature_Request__c/a2vRU000000ErFlYAK/view

Start by reviewing the AWS::RDS::DBCluster and AWS::RDS::DBInstance behavior described here, focusing on PreferredBackupWindow, BackupRetentionPeriod, AWS Backup, and PITR backups. Reproduce the reported drift scenario and determine the expected behavior when AWS Backup manages PITR; done means the requested configuration no longer produces misleading drift errors.

Written by the indexing model from the issue text.