kcm: use ContextualHelpButton from Kirigami
The old KCM version is deprecated, and all the other ones in this page are already using the Kirigami version. Noticed because the KCM version has a smaller icon, and it looked weird!
I'm 99% sure removing the KCM
import is fine, and I didn't see any issues in testing, but would appreciate a spot-check on this.