chore(rds): add support for aurora-postgresql 16.3

[Andrwe]

Reason for this change

Add support for newly supported aurora-postgresql16 16.3.

Description of changes

Add a new version as a new property to AuroraPostgresEngineVersion class. (see https://docs.aws.amazon.com/AmazonRDS/latest/PostgreSQLReleaseNotes/postgresql-versions.html)
The change already happened with #30242 and was reverted by #30296 as the version was not available.

Description of how you validated changes

The version 16.3 is available now:

$ aws rds describe-db-engine-versions --engine aurora-postgresql --query "DBEngineVersions[?EngineVersion=='16.3']"
[
    {
        "Engine": "aurora-postgresql",
        "EngineVersion": "16.3",
        "DBParameterGroupFamily": "aurora-postgresql16",
        "DBEngineDescription": "Aurora (PostgreSQL)",
        "DBEngineVersionDescription": "Aurora PostgreSQL (Compatible with PostgreSQL 16.3)",
        "ValidUpgradeTarget": [],
        "ExportableLogTypes": [
            "postgresql"
        ],
        "SupportsLogExportsToCloudwatchLogs": true,
        "SupportsReadReplica": false,
        "SupportedEngineModes": [
            "provisioned"
        ],
        "SupportedFeatureNames": [
            "Comprehend",
            "Lambda",
            "s3Export",
            "s3Import",
            "SageMaker"
        ],
        "Status": "available",
        "SupportsParallelQuery": false,
        "SupportsGlobalDatabases": true,
        "MajorEngineVersion": "16",
        "SupportsBabelfish": true,
        "SupportsLimitlessDatabase": false,
        "SupportsCertificateRotationWithoutRestart": true,
        "SupportedCACertificateIdentifiers": [
            "rds-ca-2019",
            "rds-ca-ecc384-g1",
            "rds-ca-rsa4096-g1",
            "rds-ca-rsa2048-g1"
        ],
        "SupportsLocalWriteForwarding": true,
        "SupportsIntegrations": false
    }
]

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

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

[aws-cdk-automation]

The pull request linter has failed. See the aws-cdk-automation comment below for failure reasons. If you believe this pull request should receive an exemption, please comment and provide a justification.

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[Andrwe]

Exemption Request

I request an exemption from the rules regarding README and integration tests as this PR only adds support for a new version of AuroraPostgresEngineVersion to be up-to-date to versions supported by AWS Aurora service.
Therefore no functionality is added, changed or removed that IMHO needs additional testing.

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: e5cc57f
• Result: SUCCEEDED
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