Update cobertura for gitlab 15
Adam Mercer reports:
It turns out we were effected by some of the breaking changes in GitLab-15. Notably the artifacts:reports:cobertura keyword was removed and replaced by artifacts:reports:coverage_report.
https://docs.gitlab.com/15.0/ee/ci/yaml/artifacts_reports.html#artifactsreportscobertura-removed
This MR fixes this issue