RDS: Is it possible to specify LogGroup RetentionDays for ServerlessCluster?

### Describe the feature

I can only see the logs for current day for my ServerlessCluster. What is the default days for its LogGroup?

`/aws/rds/cluster/???/postgresql`

### Use Case

Would like the ability to retain the LogGroup for a certain period of time for auditing purpose.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

1.140.0

### Environment details (OS name and version, etc.)

n/a

Start by reading the ServerlessCluster construct and checking how its /aws/rds/cluster/???/postgresql log group is created and what retention it currently receives. Confirm the current default and determine the requested retention behavior for auditing; the issue does not identify a source file or test, so those must be located before implementation.

Written by the indexing model from the issue text.