feat(rds): add missing aurora postgres versions (#20830)

Add new Postgres versions which are already available in AWS. 

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: wilhen01

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

@@ -501,6 +501,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_10_19 = AuroraPostgresEngineVersion.of('10.19', '10', { s3Import: true, s3Export: true });
   /** Version "10.20". */
   public static readonly VER_10_20 = AuroraPostgresEngineVersion.of('10.20', '10', { s3Import: true, s3Export: true });
+  /** Version "10.21". */
+  public static readonly VER_10_21 = AuroraPostgresEngineVersion.of('10.21', '10', { s3Import: true, s3Export: true });
   /** Version "11.4". */
   public static readonly VER_11_4 = AuroraPostgresEngineVersion.of('11.4', '11', { s3Import: true });
   /** Version "11.6". */
@@ -519,6 +521,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_11_14 = AuroraPostgresEngineVersion.of('11.14', '11', { s3Import: true, s3Export: true });
   /** Version "11.15". */
   public static readonly VER_11_15 = AuroraPostgresEngineVersion.of('11.15', '11', { s3Import: true, s3Export: true });
+  /** Version "11.16". */
+  public static readonly VER_11_16 = AuroraPostgresEngineVersion.of('11.16', '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". */
@@ -529,6 +533,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_12_9 = AuroraPostgresEngineVersion.of('12.9', '12', { s3Import: true, s3Export: true });
   /** Version "12.10". */
   public static readonly VER_12_10 = AuroraPostgresEngineVersion.of('12.10', '12', { s3Import: true, s3Export: true });
+  /** Version "12.11". */
+  public static readonly VER_12_11 = AuroraPostgresEngineVersion.of('12.11', '12', { s3Import: true, s3Export: true });
   /** Version "13.3". */
   public static readonly VER_13_3 = AuroraPostgresEngineVersion.of('13.3', '13', { s3Import: true, s3Export: true });
   /** Version "13.4". */
@@ -537,6 +543,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_13_5 = AuroraPostgresEngineVersion.of('13.5', '13', { s3Import: true, s3Export: true });
   /** Version "13.6". */
   public static readonly VER_13_6 = AuroraPostgresEngineVersion.of('13.6', '13', { s3Import: true, s3Export: true });
+  /** Version "13.7". */
+  public static readonly VER_13_7 = AuroraPostgresEngineVersion.of('13.7', '13', { s3Import: true, s3Export: true });
 
   /**
    * Create a new AuroraPostgresEngineVersion with an arbitrary version.