running: Update for Debian 11
running: Update for Debian 11

Debian 10 already has cockpit in the release (although at version 188).
So update the instructions for both 10 and 11 to clarify the backports
situation and recommendation.

Fixes #444

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