[sddm-theme] Update theme preview with 5.16 wallpaper

Summary: Our current preview has become inaccurate so this patch updates it.

Reviewers: GB_2, ngraham, #vdg

Reviewed By: ngraham, #vdg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21560