chore(rds): add support for postgres 16.3 engine (#30242)

### Issue # (if applicable)

N/A

### Reason for this change

RDS can be upgraded/created to Postgres 16.3 via the console/CLI but not CDK.

### Description of changes

Added Postgres 16.3 support for RDS clusters and instances. Updated examples to use Postgres 16.3.

### Description of how you validated changes

Tests have been updated to use Postgres 16.3.

### 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*

packages/@aws-cdk-testing/framework-integ/test/aws-rds/test/integ.instance-dual.ts

@@ -15,7 +15,7 @@ vpc.isolatedSubnets.forEach((subnet, idx) => {
 });
 
 new rds.DatabaseInstance(stack, 'DualstackInstance', {
-  engine: rds.DatabaseInstanceEngine.postgres({ version: rds.PostgresEngineVersion.VER_16_2 }),
+  engine: rds.DatabaseInstanceEngine.postgres({ version: rds.PostgresEngineVersion.VER_16_3 }),
   credentials: rds.Credentials.fromUsername('postgres', { password: cdk.SecretValue.unsafePlainText('7959866cacc02c2d243ecfe177464fe6') }),
   instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE3, ec2.InstanceSize.MICRO),
   vpc,
@@ -27,7 +27,7 @@ new rds.DatabaseInstance(stack, 'DualstackInstance', {
 });
 
 new rds.DatabaseInstance(stack, 'Ipv4Instance', {
-  engine: rds.DatabaseInstanceEngine.postgres({ version: rds.PostgresEngineVersion.VER_16_2 }),
+  engine: rds.DatabaseInstanceEngine.postgres({ version: rds.PostgresEngineVersion.VER_16_3 }),
   credentials: rds.Credentials.fromUsername('postgres', { password: cdk.SecretValue.unsafePlainText('7959866cacc02c2d243ecfe177464fe6') }),
   instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE3, ec2.InstanceSize.MICRO),
   vpc,