chore(neptune): add neptune engine version 1.3.1.0 (aws#30169)

### Description of changes



### Description of how you validated changes
Add engine version.

https://docs.aws.amazon.com/neptune/latest/userguide/engine-releases-1.3.1.0.html


### 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)

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*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-neptune-alpha/lib/cluster.ts

@@ -80,6 +80,10 @@ export class EngineVersion {
    * Neptune engine version 1.3.0.0
    */
   public static readonly V1_3_0_0 = new EngineVersion('1.3.0.0');
+  /**
+   * Neptune engine version 1.3.1.0
+   */
+  public static readonly V1_3_1_0 = new EngineVersion('1.3.1.0');
 
   /**
    * Constructor for specifying a custom engine version