aws-rds: defaultDatabaseName name constrain change caused database replacement.

### Describe the bug

When updating DatabaseCluster with defaultDatabaseName value `${envName}-db` caused error "DatabaseName must begin with a letter and contain only alphanumeric characters".
We where forced change code into " defaultDatabaseName: `${envName}_db`," and doing some data recovery hurdles.

I think CDK has been doing defaultDatabaseName value sanitatiotion and now this functionality is removed.

### Regression Issue

- [x] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

2.234.1

### Expected Behavior

Name with "-" should be allowed.

### Current Behavior

Cannot use "-" symbol in database name.

### Reproduction Steps

Create aurora db on version 2.234.1 with defaultDatabaseName containning "-" in name.
CDK allow database creation.
Updated newer CDK version.
Try to update stack and get errro "DatabaseName must begin with a letter and contain only alphanumeric characters.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.234.1

### AWS CDK CLI version

--

### Node.js Version

24

### OS

Windows

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at the aws-rds DatabaseCluster handling of defaultDatabaseName and compare behavior between CDK 2.234.1 and the newer version described. Reproduce an update using an envName-db value and verify that the generated database name remains accepted without database replacement or recovery work.

Written by the indexing model from the issue text.