Removal of artifacts:report:cobertura keyword

Problem to solve

In &6900, we specify the removal of the artifacts:report:cobertura keyword.

Proposal

Remove artifacts:report:cobertura keyword:

• cobertura keyword: https://gitlab.com/gitlab-org/gitlab/blob/18cbbc260912e5a50f507659c53a08df7ac711f1/lib/gitlab/ci/config/entry/reports.rb#L48-L48
• validation across cobertura and coverage_report: https://gitlab.com/gitlab-org/gitlab/blob/18cbbc260912e5a50f507659c53a08df7ac711f1/lib/gitlab/ci/config/entry/reports.rb#L55-L55.

Workaround after removal

If you are coming here to see that this feature has been removed and are wondering what the replacement keyword is, here is a new example:

artifacts:
  reports:
    coverage_report:
      coverage_format: cobertura
      path: coverage/cobertura-coverage.xml