chore(rds): support RDS for PostgreSQL 16.1 and 15.5 (#28097)

This PR supports RDS for PostgreSQL 16.1 and 15.5.

```bash
❯ aws rds describe-db-engine-versions \
∙   --engine postgres \
∙   --query "DBEngineVersions[?EngineVersion=='16.1'||EngineVersion=='15.5'].[DBEngineVersionDescription,EngineVersion,DBParameterGroupFamily,MajorEngineVersion,Status]"

[
    [
        "PostgreSQL 15.5-R1",
        "15.5",
        "postgres15",
        "15",
        "available"
    ],
    [
        "PostgreSQL 16.1-R1",
        "16.1",
        "postgres16",
        "16",
        "available"
    ]
]
```

https://aws.amazon.com/about-aws/whats-new/2023/11/amazon-rds-postgresql-major-version-16/

----

*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

@@ -1276,6 +1276,13 @@ export class PostgresEngineVersion {
   public static readonly VER_15_3 = PostgresEngineVersion.of('15.3', '15', { s3Import: true, s3Export: true });
   /** Version "15.4". */
   public static readonly VER_15_4 = PostgresEngineVersion.of('15.4', '15', { s3Import: true, s3Export: true });
+  /** Version "15.5". */
+  public static readonly VER_15_5 = PostgresEngineVersion.of('15.5', '15', { s3Import: true, s3Export: true });
+
+  /** Version "16" (only a major version, without a specific minor version). */
+  public static readonly VER_16 = PostgresEngineVersion.of('16', '16', { s3Import: true, s3Export: true });
+  /** Version "16.1". */
+  public static readonly VER_16_1 = PostgresEngineVersion.of('16.1', '16', { s3Import: true, s3Export: true });
 
   /**
    * Create a new PostgresEngineVersion with an arbitrary version.