AWS::OpenSearchServerless::CollectionGroup - [Coverage] - Generation support

### Name of the resource

Other

### Resource name

AWS::OpenSearchServerless::CollectionGroup

### Description

The OpenSearch Serverless CreateCollectionGroup API now supports a new property, `generation`.

Currently, collection groups default to the `CLASSIC` generation. The CloudFormation AWS::OpenSearchServerless::CollectionGroup resource should support specifying the generation to create `CLASSIC` and `NEXTGEN` serverless collection groups.

### Other Details

- [OpenSearch Serverless CreateCollectionGroup](https://docs.aws.amazon.com/opensearch-service/latest/ServerlessAPIReference/API_CreateCollectionGroup.html#opensearchserverless-CreateCollectionGroup-request-generation)
- https://aws.amazon.com/blogs/big-data/the-next-generation-of-amazon-opensearch-serverless-built-from-the-ground-up-for-agents/

Start with the AWS::OpenSearchServerless::CollectionGroup resource coverage and the linked CreateCollectionGroup API documentation, focusing on the new generation property and its CLASSIC and NEXTGEN values. Done means the CloudFormation resource can specify generation when creating collection groups and preserves the existing default behavior.

Written by the indexing model from the issue text.