rds: add AuroraPostgresEngineVersion.VER_18_4

### Describe the feature

[Amazon Aurora supports PostgreSQL 18.4](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraPostgreSQLReleaseNotes/AuroraPostgreSQL.Updates.html#aurorapostgresql-versions-version184x)

Static constant rds.AuroraPostgresEngineVersion.VER_18_4 should be added.

### Use Case

A static constant would make implementation equally simple compared to earlier supported versions.

### Proposed Solution

Add version 18.4 to rds.AuroraPostgresEngineVersion.

### 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)

2.268.0

### AWS CDK CLI version

2.1140.0 (build 5b77563)

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

MacOS 26.6.2

Start at the rds.AuroraPostgresEngineVersion definition and compare the existing version constants. Add the VER_18_4 entry for Aurora PostgreSQL 18.4, then verify the surrounding RDS tests or generated output according to the repository’s existing pattern. Done means the static constant is available alongside the earlier supported versions.

Written by the indexing model from the issue text.