chore(rds): add Aurora Postgres engine versions 10.16 and 11.11 (#15781)

Amazon Aurora is available in 10.16 and 11.11, but I added it because it was not available in the CDK.
https://aws.amazon.com/about-aws/whats-new/2021/06/amazon-aurora-supports-postgresql-12-6-11-11-10-16-and-9-6-21/

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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

@@ -447,6 +447,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_10_13 = AuroraPostgresEngineVersion.of('10.13', '10', { s3Import: true, s3Export: true });
   /** Version "10.14". */
   public static readonly VER_10_14 = AuroraPostgresEngineVersion.of('10.14', '10', { s3Import: true, s3Export: true });
+  /** Version "10.16". */
+  public static readonly VER_10_16 = AuroraPostgresEngineVersion.of('10.16', '10', { s3Import: true, s3Export: true });
   /** Version "11.4". */
   public static readonly VER_11_4 = AuroraPostgresEngineVersion.of('11.4', '11', { s3Import: true });
   /** Version "11.6". */
@@ -457,6 +459,8 @@ 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 "11.11". */
+  public static readonly VER_11_11 = AuroraPostgresEngineVersion.of('11.11', '11', { s3Import: true, s3Export: true });
   /** Version "12.4". */
   public static readonly VER_12_4 = AuroraPostgresEngineVersion.of('12.4', '12', { s3Import: true, s3Export: true });
   /** Version "12.6". */