running: Clarify CentOS versions
running: Clarify CentOS versions

Use the same "and later" as for RHEL, to clarify that Cockpit is not
only in version 7.

martinpitt/cockpit-project.github.io,martinpitt/cockpit-project.github.io,martinpitt/cockpit-project.github.io,martinpitt/cockpit-project.github.io