chore(rds): add Aurora Postgres engine version 16.1 (#29110)

### Reason for this change

Aurora PostgreSQL 16.1 is available as of January 31, 2024. See https://docs.aws.amazon.com/AmazonRDS/latest/AuroraPostgreSQLReleaseNotes/AuroraPostgreSQL.Updates.html#AuroraPostgreSQL.Updates.20180305.161 for more information.

### Description of changes

Add constant for newly available Aurora Postgres cluster engine version 16.1

### Description of how you validated changes

No. Happy to add, if required.

### 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: robdasilva

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

@@ -918,10 +918,10 @@ 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 "16.0".
-   * Version 16.0 is available in preview environment as of November 15, 2023.
-   */
+  /** Version "16.0". */
   public static readonly VER_16_0 = AuroraPostgresEngineVersion.of('16.0', '16', { s3Import: true, s3Export: true });
+  /** Version "16.1". */
+  public static readonly VER_16_1 = AuroraPostgresEngineVersion.of('16.1', '16', { s3Import: true, s3Export: true });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.