[Feature Request] Change default values for OpenSearch redundancy and Bedrock model

## Describe the solution you'd like

- OpenSearch
- Change default value of [enalbeRagReplicas](https://github.com/aws-samples/bedrock-claude-chat/blob/v2/cdk/cdk.json#L71) as `false`
- PoC demand is still dominant & redundancy might be not necessary for most use-cases
- Also need to modify [README](https://github.com/aws-samples/bedrock-claude-chat?tab=readme-ov-file#configure-rag-replicas)

- LLM Model
- Change default [state](https://github.com/aws-samples/bedrock-claude-chat/blob/v2/frontend/src/hooks/useModel.ts#L21) to ’claude-v3.5-haiku`
- Current default model is Haiku 3, which is old version

## Why the solution needed

To adapt actual demands.

## Implementation feasibility

Are you willing to collaborate with us to discuss the solution, decide on the approach, and assist with the implementation?

- [x] Yes, I am able to implement the feature and create a pull request.
- [ ] No, I am unable to implement the feature, but I am open to discussing the solution.

Update the OpenSearch replica default in cdk/cdk.json at the linked setting, the model default in frontend/src/hooks/useModel.ts, and the README's Configure RAG Replicas section. Start by checking those referenced locations and existing configuration wording; done means both defaults use the requested values and the README matches the replica behavior.

Written by the indexing model from the issue text.