feat(rds): PostgreSQL engine version 15.2 (#24463)

RDS supports PostgreSQL engine version 15.2, but the `PostgresEngineVersion.VER_15_2` constant  is not implemented yet. The current CDK version only lists up to `VER_14_6`.

```sh
aws rds describe-db-engine-versions --engine postgres | jq -r '.DBEngineVersions[] | [.EngineVersion]' | grep "15\."
  "15.2"
```

### Use Case

```js
new DatabaseInstance(this, 'PostgresDatabase',  
  engine: DatabaseInstanceEngine.postgres({ version: PostgresEngineVersion.VER_15_2 }),
)
```

Closes #24462.

----

*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/instance-engine.ts

@@ -1049,6 +1049,8 @@ export class PostgresEngineVersion {
 
   /** Version "15" (only a major version, without a specific minor version). */
   public static readonly VER_15 = PostgresEngineVersion.of('15', '15', { s3Import: true, s3Export: true });
+  /** Version "15.2". */
+  public static readonly VER_15_2 = PostgresEngineVersion.of('15.2', '15', { s3Import: true, s3Export: true });
 
   /**
    * Create a new PostgresEngineVersion with an arbitrary version.