chore(rds): modify the ACU limit for serverless v2 from 128 to 256 (#31648)

### Issue # (if applicable)

N/A

### Reason for this change
Currently, the ACU limit is 128 on CDK, but since it now supports up to 256.

Ref: [Amazon Aurora Serverless v2 now supports up to 256 ACUs](https://aws.amazon.com/about-aws/whats-new/2024/10/amazon-aurora-serverless-v2-256-acus/)



### Description of changes
Modify the ACU limit for serverless v2 from 128 to 256.



### Description of how you validated changes
Modify unit tests and add an integ test.


### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36