Update clean-css requirement from 4.1.9 to 4.2.1 (#11891)
Update clean-css requirement from 4.1.9 to 4.2.1 (#11891)

Update clean-css requirement from 4.1.9 to 4.2.1
psiinon/addons-server,psiinon/addons-server,wagnerand/addons-server,eviljeff/olympia,diox/olympia,mozilla/olympia,mozilla/addons-server,diox/olympia,mozilla/addons-server,eviljeff/olympia,mozilla/addons-server,diox/olympia,wagnerand/addons-server,diox/olympia,bqbn/addons-server,eviljeff/olympia,eviljeff/olympia,mozilla/olympia,bqbn/addons-server,wagnerand/addons-server,mozilla/addons-server,bqbn/addons-server,mozilla/olympia,psiinon/addons-server,wagnerand/addons-server,mozilla/olympia,bqbn/addons-server,psiinon/addons-server