chore(rds): add postgres engine 16.8, 15.12, 14.17 and 13.20 (#34151)

### Issue # (if applicable)

None

### Reason for this change
https://aws.amazon.com/about-aws/whats-new/2025/04/amazon-aurora-postgresql-versions/

### Description of how you validated changes
```console
$ aws rds describe-db-engine-versions --engine postgres --output table --query 'DBEngineVersions[*].{Engine:Engine,EngineVersion:EngineVersion}'
|  postgres |  13.20               |
|  postgres |  14.12               |
|  postgres |  14.13               |
|  postgres |  14.15               |
|  postgres |  14.17               |
|  postgres |  15.7                |
|  postgres |  15.8                |
|  postgres |  15.10               |
|  postgres |  15.12               |
|  postgres |  16.3                |
|  postgres |  16.4                |
|  postgres |  16.6                |
|  postgres |  16.8                |
```

### Checklist
- [x] 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*

Author: phuhung273

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

@@ -1030,6 +1030,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_13_17 = AuroraPostgresEngineVersion.of('13.17', '13', { s3Import: true, s3Export: true });
   /** Version "13.18". */
   public static readonly VER_13_18 = AuroraPostgresEngineVersion.of('13.18', '13', { s3Import: true, s3Export: true });
+  /** Version "13.20". */
+  public static readonly VER_13_20 = AuroraPostgresEngineVersion.of('13.20', '13', { s3Import: true, s3Export: true });
   /** Version "14.3". */
   public static readonly VER_14_3 = AuroraPostgresEngineVersion.of('14.3', '14', { s3Import: true, s3Export: true });
   /**  Version "14.4". */
@@ -1056,6 +1058,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_14_14 = AuroraPostgresEngineVersion.of('14.14', '14', { s3Import: true, s3Export: true });
   /** Version "14.15". */
   public static readonly VER_14_15 = AuroraPostgresEngineVersion.of('14.15', '14', { s3Import: true, s3Export: true });
+  /** Version "14.17". */
+  public static readonly VER_14_17 = AuroraPostgresEngineVersion.of('14.17', '14', { s3Import: true, s3Export: true });
   /** Version "15.2". */
   public static readonly VER_15_2 = AuroraPostgresEngineVersion.of('15.2', '15', { s3Import: true, s3Export: true });
   /** Version "15.3". */
@@ -1074,6 +1078,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_15_9 = AuroraPostgresEngineVersion.of('15.9', '15', { s3Import: true, s3Export: true });
   /** Version "15.10". */
   public static readonly VER_15_10 = AuroraPostgresEngineVersion.of('15.10', '15', { s3Import: true, s3Export: true });
+  /** Version "15.12". */
+  public static readonly VER_15_12 = AuroraPostgresEngineVersion.of('15.12', '15', { s3Import: true, s3Export: true });
   /**
    * Version "16.0"
    * @deprecated Version 16.0 is no longer supported by Amazon RDS.
@@ -1095,6 +1101,8 @@ export class AuroraPostgresEngineVersion {
   public static readonly VER_16_6 = AuroraPostgresEngineVersion.of('16.6', '16', { s3Import: true, s3Export: true });
   /** Version "16.6 limitless" */
   public static readonly VER_16_6_LIMITLESS = AuroraPostgresEngineVersion.of('16.6-limitless', '16', { s3Import: true, s3Export: true });
+  /** Version "16.8". */
+  public static readonly VER_16_8 = AuroraPostgresEngineVersion.of('16.8', '16', { s3Import: true, s3Export: true });
   /** Version "17.1". */
   public static readonly VER_17_1 = AuroraPostgresEngineVersion.of('17.1', '17', { s3Import: true, s3Export: true });
   /** Version "17.2". */

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

@@ -53,6 +53,8 @@ describe('cluster engine', () => {
       'aurora-postgresql9.6');
     expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('10.0', '10') }).parameterGroupFamily).toEqual(
       'aurora-postgresql10');
+    expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('13.20', '13') }).parameterGroupFamily).toEqual(
+      'aurora-postgresql13');
     expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('14.3', '14') }).parameterGroupFamily).toEqual(
       'aurora-postgresql14');
     expect(DatabaseClusterEngine.auroraPostgres({ version: AuroraPostgresEngineVersion.of('15.7', '15') }).parameterGroupFamily).toEqual(
@@ -70,16 +72,28 @@ describe('cluster engine', () => {
 
   test('cluster parameter group correctly determined for AURORA_POSTGRESQL and given version', () => {
     // GIVEN
+    const engine_VER_13_20 = DatabaseClusterEngine.auroraPostgres({
+      version: AuroraPostgresEngineVersion.VER_13_20,
+    });
     const engine_VER_14_3 = DatabaseClusterEngine.auroraPostgres({
       version: AuroraPostgresEngineVersion.VER_14_3,
     });
+    const engine_VER_15_12 = DatabaseClusterEngine.auroraPostgres({
+      version: AuroraPostgresEngineVersion.VER_15_12,
+    });
     const engine_VER_16_3 = DatabaseClusterEngine.auroraPostgres({
       version: AuroraPostgresEngineVersion.VER_16_3,
     });
+    const engine_VER_17_2 = DatabaseClusterEngine.auroraPostgres({
+      version: AuroraPostgresEngineVersion.VER_17_2,
+    });
 
     // THEN
+    expect(engine_VER_13_20.parameterGroupFamily).toEqual('aurora-postgresql13');
     expect(engine_VER_14_3.parameterGroupFamily).toEqual('aurora-postgresql14');
+    expect(engine_VER_15_12.parameterGroupFamily).toEqual('aurora-postgresql15');
     expect(engine_VER_16_3.parameterGroupFamily).toEqual('aurora-postgresql16');
+    expect(engine_VER_17_2.parameterGroupFamily).toEqual('aurora-postgresql17');
   });
 
   testDeprecated('cluster parameter group correctly determined for AURORA and given version', () => {