(glue-alpha): support AWS Glue version 6.0

### Describe the feature

Add `GlueVersion.V6_0` to the `GlueVersion` enum in `@aws-cdk/aws-glue-alpha` (Spark 4.1.1, Python 3.13, Scala 2.13).

AWS Glue 6.0 is now generally available:
https://aws.amazon.com/about-aws/whats-new/2026/08/aws-glue-6-0-price-reduction-iceberg-v3/

### Use Case

Glue jobs can already be configured with `glueVersion: '6.0'` via the L1 `CfnJob`, but the L2 `GlueVersion` enum has no type-safe constant for it yet.

### Proposed Solution

Add a `V6_0 = '6.0'` member to `GlueVersion` in `lib/constants.ts`, matching the pattern used for 5.0/5.1.

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Open lib/constants.ts in @aws-cdk/aws-glue-alpha and read the existing GlueVersion members for 5.0 and 5.1. Add the 6.0 enum member following that pattern; done means the L2 exposes a type-safe GlueVersion.V6_0 value of '6.0'.

Written by the indexing model from the issue text.