chore(rds): support 11.22-rds.20240808 for RDS PostgreSQL (aws#31233)

Add new instance engine.

Ref: [Amazon RDS for PostgreSQL announces Extended Support minor 11.22-RDS.20240808](https://aws.amazon.com/about-aws/whats-new/2024/08/amazon-rds-postgresql-minor-11-22-rds-20240808/)

```sh
% aws rds describe-db-engine-versions --engine postgres --query "DBEngineVersions[].EngineVersion" --output table --region us-east-1
--------------------------
|DescribeDBEngineVersions|
+------------------------+
// omit
|  11.22-rds.20240808    |
// omit
```

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

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

@@ -1449,6 +1449,10 @@ export class PostgresEngineVersion {
    * Version "11.22-rds.20240509"
    */
   public static readonly VER_11_22_RDS_20240509 = PostgresEngineVersion.of('11.22-rds.20240509', '11', { s3Import: true, s3Export: true });
+  /**
+   * Version "11.22-rds.20240808"
+   */
+  public static readonly VER_11_22_RDS_20240808 = PostgresEngineVersion.of('11.22-RDS.20240808', '11', { s3Import: true, s3Export: true });
 
   /** Version "12" (only a major version, without a specific minor version). */
   public static readonly VER_12 = PostgresEngineVersion.of('12', '12', { s3Import: true });