[SDDM theme] Use the biggest wallpaper resolution
Summary: Starting from Plasma 5.16 we have the Next wallpaper is available in 5K resolution so we can make use of it in the login screen as well.
Reviewed By: VDG, ngraham
Subscribers: davidedmundson, ngraham, plasma-devel
Maniphest Tasks: T10269
Differential Revision: https://phabricator.kde.org/D18007