chore(rds): add support for postgres 15.6 aurora engine (#30167)

### Issue # (if applicable)

N/A

### Reason for this change

Cluster can be upgraded/created to Postgres 15.6 via the console/CLI but not CDK.

### Description of changes

Adds support for Postgres 15.6 Aurora cluster, 15.5 instances are already supported.

### Description of how you validated changes

N/A

### 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)

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

Author: Chemsmith

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

@@ -1002,6 +1002,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_15_4 = AuroraPostgresEngineVersion.of('15.4', '15', { s3Import: true, s3Export: true });
   /** Version "15.5". */
   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 "16.0"
    * @deprecated Version 16.0 is no longer supported by Amazon RDS.