Outdated GTK version specified in mousepad-prefs-dialog.ui

https://gitlab.xfce.org/apps/mousepad/-/blob/master/mousepad/resources/ui/mousepad-prefs-dialog.ui#L3

Another thing fixed in a pending MR, which I don't think should change much, but okay, let's fix it now.

closed with commit 443ce29f

changed milestone to %0.5.0

assigned to @Tamaranch