feat: migrate mixtral for code generations

What does this merge request do and why?

Add mixtral prompt for code generation (same as mistral).

Part of gitlab-org/gitlab#470822 (closed).

How to set up and validate locally

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

Merge request checklist

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