Remove of artifacts:report:cobertura keyword

What does this MR do and why?

Remove of artifacts:report:cobertura keyword as it has been replaced with artifacts:report:coverage_report.

This MR removes the cobertura keyword in CI config, the CI schema used by the pipeline editor, and documentation related to the keyword.

Related to #348980 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.