docs: Remove Stretch from install docs
docs: Remove Stretch from install docs

The Stretch dist still exists on apt.mopidy.com, but we don't build new
packages for it any longer.

jodal/mopidy,jcass77/mopidy,mopidy/mopidy,adamcik/mopidy,jcass77/mopidy,mopidy/mopidy,adamcik/mopidy,kingosticks/mopidy,kingosticks/mopidy,jodal/mopidy,jodal/mopidy,kingosticks/mopidy,mopidy/mopidy,adamcik/mopidy,jcass77/mopidy