krunner: Store pinned state in krunnerstaterc and not in config

The krunnerstaterc file is also used in the framework, see https://invent.kde.org/frameworks/krunner/-/blob/master/src/runnermanager.cpp#L559 and the motivation for this MR is described in https://phabricator.kde.org/T12549.

Considering that it is just one small value and not any user critical setting I don't think we need to migrate existing entries.