feat(rds): support Aurora PostgreSQL Limitless Database PostgreSQL 16.6 (#33162)

Add new Limitless Database Engine.

Ref:
* https://aws.amazon.com/about-aws/whats-new/2025/01/amazon-aurora-postgresql-limitless-database-16-6/
* https://docs.aws.amazon.com/AmazonRDS/latest/AuroraPostgreSQLReleaseNotes/limitless-updates.html#16.6-limitless

### 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: mazyu36

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

@@ -1087,6 +1087,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_16_5 = AuroraPostgresEngineVersion.of('16.5', '16', { s3Import: true, s3Export: true });
   /** Version "16.6". */
   public static readonly VER_16_6 = AuroraPostgresEngineVersion.of('16.6', '16', { s3Import: true, s3Export: true });
+  /** Version "16.6 limitless" */
+  public static readonly VER_16_6_LIMITLESS = AuroraPostgresEngineVersion.of('16.6-limitless', '16', { s3Import: true, s3Export: true });
   /** Version "17.1". */
   public static readonly VER_17_1 = AuroraPostgresEngineVersion.of('17.1', '17', { s3Import: true, s3Export: true });
   /** Version "17.2". */