Cobertura plugin page and readme should announce deprecation.

Cobertura depends on the deprecated Code Coverage plugin, which asks users to migrate to the Coverage plugin, which supports Cobertura.

See

• https://plugins.jenkins.io/cobertura/
• https://plugins.jenkins.io/code-coverage-api/
• https://plugins.jenkins.io/coverage/

See also https://github.com/jenkinsci/cobertura-plugin/issues/226