Change model for ci cd tool to Claude 2.1

What does this MR do and why?

Start using newer model, Claude 2.1 for CI/CD tool for chat.

https://docs.anthropic.com/claude/docs/how-to-use-system-prompts

This MR also contains code that makes it possible to switch to the new prompt system easily.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Setup Duo Chat
2. Ask chat about ci/cd or run real request tests
3. Observe llm.log and see how request is send to Claude 2.1.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.