chore(neptune): support ver 1.4.6.0 and 1.4.6.1 (#35786)

Ref: [Engine releases for Amazon Neptune](https://docs.aws.amazon.com/neptune/latest/userguide/engine-releases.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)

----

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

Author: mazyu36

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

@@ -138,7 +138,14 @@ export class EngineVersion {
    * Neptune engine version 1.4.5.1
    */
   public static readonly V1_4_5_1 = new EngineVersion('1.4.5.1');
-
+  /**
+   * Neptune engine version 1.4.6.0
+   */
+  public static readonly V1_4_6_0 = new EngineVersion('1.4.6.0');
+  /**
+   * Neptune engine version 1.4.6.1
+   */
+  public static readonly V1_4_6_1 = new EngineVersion('1.4.6.1');
   /**
    * Constructor for specifying a custom engine version
    * @param version the engine version of Neptune