chore(rds): add additional AuroraPostgresEngineVersion 11.9 (aws#12267)

closes aws#12266

This change adds support for `AuroraPostgresEngineVersion` version 11.9 used for db.r6g instance types Aurora.

https://aws.amazon.com/about-aws/whats-new/2020/12/introducing-amazon-aurora-r6g-instance-types-powered-by-aws-graviton-2-processors-in-preview/

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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

@@ -442,6 +442,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_11_7 = AuroraPostgresEngineVersion.of('11.7', '11', { s3Import: true, s3Export: true });
   /** Version "11.8". */
   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 });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.