[Cursors KCM] Use new icon

Summary:
Use the nice new icon added for this KCM in D18686. It landed in Frameworks 5.56,
but that shouldn't be a problem since Plasma 5.16 is scheduled to depend on 5.58.

Test Plan:

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18687