chore(neptune): engine version 1.3.4.0 (#31989)

### Issue # (if applicable)

None

### Reason for this change

Neptune had released engine version 1.3.4.0.

https://docs.aws.amazon.com/neptune/latest/userguide/engine-releases-1.3.4.0.html

### Description of changes

Add `v1_3_4_0` member to `EngineVersion` class

### Description of how you validated changes

None

### 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: badmintoncryer

packages/@aws-cdk/aws-neptune-alpha/lib/cluster.ts

@@ -104,6 +104,10 @@ export class EngineVersion {
    * Neptune engine version 1.3.3.0
    */
   public static readonly V1_3_3_0 = new EngineVersion('1.3.3.0');
+  /**
+   * Neptune engine version 1.3.4.0
+   */
+  public static readonly V1_3_4_0 = new EngineVersion('1.3.4.0');
 
   /**
    * Constructor for specifying a custom engine version