aws-rds: Add support for Aurora PostgreSQL 18.3

### Describe the feature

## [Amazon Aurora now supports PostgreSQL major version 18](https://aws.amazon.com/about-aws/whats-new/2026/06/amazon-aurora-postgresql-major-version-18/)

Per AWS announcement, Aurora PostgreSQL now supports version 18.3. Add support in the CDK lib.

### Use Case

I wish to upgrade my CDK-managed PostgreSQL clusters to version 18.3.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

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

2.260.0

### AWS CDK CLI version

2.1121.0

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

macOS Taho 26.5.1

Start in the aws-rds CDK library and trace how supported Aurora PostgreSQL engine versions are defined and tested. Add version 18.3 support and update the relevant tests so CDK-managed Aurora PostgreSQL clusters can select it successfully.

Written by the indexing model from the issue text.