chore(rds): add support for PostgreSQL 14 (aws#18713)

Amazon RDS for PostgreSQL supports now a major version 14.

https://aws.amazon.com/about-aws/whats-new/2022/01/amazon-rds-postgresql-14-outposts/

----

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

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

@@ -903,6 +903,11 @@ export class PostgresEngineVersion {
   /** Version "13.5". */
   public static readonly VER_13_5 = PostgresEngineVersion.of('13.5', '13', { s3Import: true, s3Export: true });
 
+  /** Version "14" (only a major version, without a specific minor version). */
+  public static readonly VER_14 = PostgresEngineVersion.of('14', '14', { s3Import: true, s3Export: true });
+  /** Version "14.1". */
+  public static readonly VER_14_1 = PostgresEngineVersion.of('14.1', '14', { s3Import: true, s3Export: true });
+
   /**
    * Create a new PostgresEngineVersion with an arbitrary version.
    *