sddm-theme: switch to png format

In Plasma 5.26 the default wallpaper format has been changed to png, so the
image path in the config file of the default sddm theme also needs to be
changed.


Requires: breeze!256


(cherry picked from commit edd2516f)

lookandfeel/sddm-theme/theme.conf.cmake

@@ -4,5 +4,5 @@ logo=${KDE_INSTALL_FULL_DATADIR}/sddm/themes/breeze/default-logo.svg
 type=image
 color=#1d99f3
 fontSize=10
-background=${KDE_INSTALL_FULL_WALLPAPERDIR}/Next/contents/images/5120x2880.jpg
+background=${KDE_INSTALL_FULL_WALLPAPERDIR}/Next/contents/images/5120x2880.png
 needsFullUserModel=false