Port to KF6-proof KParts::PartLoader::instantiatePartForMimeType

Backport of 91c42f83 ("Adapt to changes in the plugin system"), already applyable to master (though needs KF 5.100).

Beware: changed qWarning -> qCWarning, so commit in kf6 branch needs to be explicitly dropped to avoid conflicts, will not auto-resolve.