feat(react): update prompts for Duo Chat

What does this merge request do and why?

Since chat v2 endpoint was created, we added some changes to the prompt on Rails. This MR updates the prompt used in AI GW in /v2/chat/agent

This feature is behind the feature flag [Feature flag] Enable v2_chat_agent_integration (gitlab-org/gitlab#466910 - closed) and will be validated with CEF separately.

How to set up and validate locally

Run GitLab locally with enabled flag for v2 chat and ask chat questions, it should return correct answers.

Merge request checklist

• [-] Tests added for new functionality. If not, please raise an issue to follow up.
• [-] Documentation added/updated, if needed.