Add update script for new klipper clear dialog config

Previously, Klipper "clear history" dialog used to not ask again even if user answered No. This was changed so that only a Yes answer will lead to no more asking, by using warningContinueCancel instead of questionYesNo.

Now the behaviour of previous config value really_clear_history has inverted: true/undefined => ask again; false => don't ask. This update script migrates old configs to use the new config value, renamed to klipperClearHistoryAskAgain.

Depends on plasma-workspace!816 (merged).