chore(rds): add AuroraPostgresEngineVersion 12.4 (#12888)

A new version of Aurora Postgres was recently released https://aws.amazon.com/about-aws/whats-new/2021/01/amazon-aurora-supports-postgresql-12/

![Screen Shot 2021-02-05 at 9 15 58 AM](https://user-images.githubusercontent.com/5084492/107052265-0e239b00-6793-11eb-964c-bd69e64ff3b4.png)


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

@@ -455,6 +455,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_11_8 = AuroraPostgresEngineVersion.of('11.8', '11', { s3Import: true, s3Export: true });
   /** Version "11.9". */
   public static readonly VER_11_9 = AuroraPostgresEngineVersion.of('11.9', '11', { s3Import: true, s3Export: true });
+  /** Version "12.4". */
+  public static readonly VER_12_4 = AuroraPostgresEngineVersion.of('12.4', '12', { s3Import: true, s3Export: true });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.