feat: add review_merge_request prompts

What does this merge request do and why?

Add required prompt for Review Merge Request feature using Clause 3.7 as a default model

• Contributes to gitlab-org/gitlab#501321 (closed)
• Related Rails change gitlab-org/gitlab!191007 (merged)

The prompt is extracted from Rails with no introduced changes.

Merge request checklist

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