swh-pipeline: Migrate from 'Code Coverage' plugin to 'Coverage' plugin

The Code Coverage plugin is now deprecated and no longer maintained, resulting to broken coverage reports in Jenkins UI.

Fortunately, all its functionalities have been integrated into the new Coverage plugin, that superseeds it.

So migrate to the new Coverage plugin in main SWH groovy pipeline in order to get back functional code coverage reports in Jenkins UI.

I installed the new plugin in our production Jenkins, patched the swh-counters pipeline with those changes and functional coverage report is back on track.