feat(rds): support zero ACU for Aurora Serverless V2 (aws#32231)

Closes aws#32234.

Amazon Aurora Serverless v2 supports scaling to zero capacity.

Ref: https://aws.amazon.com/about-aws/whats-new/2024/11/amazon-aurora-serverless-v2-scaling-zero-capacity/

"Updated validation to permit zero as a value for `serverlessV2MinCapacity`.

Fix unit tests and add an integ test.

- [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)

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Author: mazyu36