Update prompt to review all changed diffs in one go

Goal

Provide more context in order to get more accurate review comments.

This is the next step after #510266 (closed).

Proposal

Experiment in Langsmith to add all changed files to the prompt, instead of iterating on each file.

Update the prompt in the monolith.

TBD: add a limit? We should decide based on https://gitlab.com/gitlab-org/gitlab/-/issues/532451.