feat(rds): support aurora postgresql 15.3 (aws#26377)

Add Aurora PostgreSQL 15.3.0 to database cluster engine.
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraPostgreSQLReleaseNotes/AuroraPostgreSQL.Updates.html#AuroraPostgreSQL.Updates.20180305.1530

S3 import and s3 export features are supported.
```bash
$ aws rds describe-db-engine-versions --region us-east-1 --engine aurora-postgresql --engine-version 15.3
{
    "DBEngineVersions": [
        {
            "Engine": "aurora-postgresql",
            "EngineVersion": "15.3",
            "DBParameterGroupFamily": "aurora-postgresql15",
            "DBEngineDescription": "Aurora (PostgreSQL)",
            "DBEngineVersionDescription": "Aurora PostgreSQL (Compatible with PostgreSQL 15.3)",
            "ValidUpgradeTarget": [],
            "ExportableLogTypes": [
                "postgresql"
            ],
            "SupportsLogExportsToCloudwatchLogs": true,
            "SupportsReadReplica": false,
            "SupportedEngineModes": [
                "provisioned"
            ],
            "SupportedFeatureNames": [
                "Comprehend",
                "Lambda",
                "s3Export",
                "s3Import",
                "SageMaker"
            ],
            "Status": "available",
            "SupportsParallelQuery": false,
            "SupportsGlobalDatabases": true,
            "MajorEngineVersion": "15",
            "SupportsBabelfish": true,
            "SupportsCertificateRotationWithoutRestart": true,
            "SupportedCACertificateIdentifiers": [
                "rds-ca-2019",
                "rds-ca-ecc384-g1",
                "rds-ca-rsa4096-g1",
                "rds-ca-rsa2048-g1"
            ]
        }
    ]
}
```

Closes aws#26363.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-rds/lib/cluster-engine.ts

@@ -712,6 +712,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_14_7 = AuroraPostgresEngineVersion.of('14.7', '14', { s3Import: true, s3Export: true });
   /** Version "15.2". */
   public static readonly VER_15_2 = AuroraPostgresEngineVersion.of('15.2', '15', { s3Import: true, s3Export: true });
+  /** Version "15.3". */
+  public static readonly VER_15_3 = AuroraPostgresEngineVersion.of('15.3', '15', { s3Import: true, s3Export: true });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.