Add List merge request diffs API test
Existing GET /projects/:id/merge_requests/:merge_request_iid/changes
is deprecated since 15.7 in favour of GET /projects/:id/merge_requests/:merge_request_iid/diffs
, see gitlab-org/gitlab#322117 (comment 1188807929)
The issue is to add new endpoint to GPT or update existing api_v4_projects_merge_requests_merge_request_changes
to switch to a new endpoint path when GitLab version is >= 15.7.