chore(rds): add support for aurora-postgresql 15.7

[yasuaki640]

Issue # (if applicable)

Closes #.

Reason for this change

There is no enum of 15.7.

Description of changes

Added already supported 15.7 to AuroraPostgressEngineVersion the class.
https://docs.aws.amazon.com/AmazonRDS/latest/PostgreSQLReleaseNotes/postgresql-versions.html#postgresql-version15

Description of how you validated changes

aws rds describe-db-engine-versions --engine aurora-postgresql --query "DBEngineVersions[?EngineVersion=='15.7']"
[
    {
        "Engine": "aurora-postgresql",
        "EngineVersion": "15.7",
        "DBParameterGroupFamily": "aurora-postgresql15",
        "DBEngineDescription": "Aurora (PostgreSQL)",
        "DBEngineVersionDescription": "Aurora PostgreSQL (Compatible with PostgreSQL 15.7)",
        "ValidUpgradeTarget": [
            {
                "Engine": "aurora-postgresql",
                "EngineVersion": "16.3",
                "Description": "Aurora PostgreSQL (Compatible with PostgreSQL 16.3)",
                "AutoUpgrade": false,
                "IsMajorVersionUpgrade": true,
                "SupportedEngineModes": [
                    "provisioned"
                ],
                "SupportsParallelQuery": false,
                "SupportsGlobalDatabases": true,
                "SupportsBabelfish": true,
                "SupportsLocalWriteForwarding": true
            }
        ],
        "ExportableLogTypes": [
            "postgresql"
        ],
        "SupportsLogExportsToCloudwatchLogs": true,
        "SupportsReadReplica": false,
        "SupportedEngineModes": [
            "provisioned"
        ],
        "SupportedFeatureNames": [
            "Bedrock",
            "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"
        ],
        "SupportsLocalWriteForwarding": true
    }
]

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

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 46fd59e
• Result: FAILED
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