chore(rds): add support for aurora-postgresql 15.7 (#31085)

### Issue # (if applicable)

Closes #<issue number here>.

### 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/AuroraPostgreSQLReleaseNotes/AuroraPostgreSQL.Updates.html#aurorapostgresql-versions-version15



### Description of how you validated changes



```sh
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
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*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

@@ -1016,6 +1016,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_15_5 = AuroraPostgresEngineVersion.of('15.5', '15', { s3Import: true, s3Export: true });
   /** Version "15.6". */
   public static readonly VER_15_6 = AuroraPostgresEngineVersion.of('15.6', '15', { s3Import: true, s3Export: true });
+  /** Version "15.7". */
+  public static readonly VER_15_7 = AuroraPostgresEngineVersion.of('15.7', '15', { s3Import: true, s3Export: true });
   /**
    * Version "16.0"
    * @deprecated Version 16.0 is no longer supported by Amazon RDS.

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

@@ -106,6 +106,8 @@ describe('cluster engine', () => {
       'aurora-postgresql10');
     expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('14.3', '14') }).parameterGroupFamily).toEqual(
       'aurora-postgresql14');
+    expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('15.7', '15') }).parameterGroupFamily).toEqual(
+      'aurora-postgresql15');
     expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('16.3', '16') }).parameterGroupFamily).toEqual(
       'aurora-postgresql16');
   });