feat: upgrade to Claude 4.0 Sonnet Merge Commit Message Generation

What does this merge request do and why?

reference !2078 (diffs) this should be reviewed together with gitlab-org/gitlab!193793 (merged)

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.