Deprecate usage of MergeRequest#commits without a limit

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We've encountered before the performance degradations due to fetching all commits for a merge request: #35542 (closed). The limit param has been introduced for narrowing the number of commits fetched: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/merge_request.rb#L423. The limit is optional, but we need to make sure that we avoid calling this method without it. However, we need to fix all the cases that are using this method without a limit before we can make it required