chore(rds): added Postgres 16.4 version (#31088)

### Issue # (if applicable)

Closes #31081.

### Reason for this change
Per [PostgreSQL version 16.4 on Amazon RDS](https://docs.aws.amazon.com/AmazonRDS/latest/PostgreSQLReleaseNotes/postgresql-versions.html#postgresql-versions-version164), PostgreSQL version 16.4 is now available on Amazon RDS.

Executing AWS CLI command `aws rds describe-db-engine-versions --engine postgres --output table --query 'DBEngineVersions[*].{Engine:Engine,EngineVersion:EngineVersion}'` also lists `16.4` as one of the supported versions.

### Description of changes
Added `16.4` version to [PostgresEngineVersion](https://github.com/aws/aws-cdk/blob/501a4f1a81983992ec04a1acbed84d09e9470f8c/packages/aws-cdk-lib/aws-rds/lib/instance-engine.ts#L1014C14-L1014C35) class.

### 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*

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

@@ -1669,6 +1669,8 @@ export class PostgresEngineVersion {
   public static readonly VER_16_2 = PostgresEngineVersion.of('16.2', '16', { s3Import: true, s3Export: true });
   /** Version "16.3". */
   public static readonly VER_16_3 = PostgresEngineVersion.of('16.3', '16', { s3Import: true, s3Export: true });
+  /** Version "16.4". */
+  public static readonly VER_16_4 = PostgresEngineVersion.of('16.4', '16', { s3Import: true, s3Export: true });
 
   /**
    * Create a new PostgresEngineVersion with an arbitrary version.