chore(rds): removed unreleased aurora postgres 16.3 cluster engine. (#30296)

### Issue #: 
Closes #30254.

### Reason for this change
PR #30242 added Aurora Postgres version `16.3`, which is not yet released.

### Description of changes
This PR removes the yet unreleased version of Aurora Postgres version `16.3` that was added in PR #30242.

Running `aws rds describe-db-engine-versions --engine aurora-postgresql --query '*[].[EngineVersion]' --output text` via AWS CLI does not list `16.3` version yet.

### 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)

----

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

Author: ashishdhingra

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

@@ -1007,8 +1007,6 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_16_1 = AuroraPostgresEngineVersion.of('16.1', '16', { s3Import: true, s3Export: true });
   /** Version "16.2". */
   public static readonly VER_16_2 = AuroraPostgresEngineVersion.of('16.2', '16', { s3Import: true, s3Export: true });
-  /** Version "16.3". */
-  public static readonly VER_16_3 = AuroraPostgresEngineVersion.of('16.3', '16', { s3Import: true, s3Export: true });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.