sddm-theme: Change wallpaper extension so that it points at a real file

The Plasma 5.24 wallpaper changed to a JPEG but this change had to be
reflected in the SDDM config file, which calls out the wallpaper by its
exact file path. This commit does that.

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.png
+background=${KDE_INSTALL_FULL_WALLPAPERDIR}/Next/contents/images/5120x2880.jpg
 needsFullUserModel=false