feat: Disable rounded corners for GTK windows, b=closes #6302, c=no-component

prefs/zen/gtk.yaml

@@ -4,7 +4,9 @@
 
 # GTK-specific preferences
 - name: widget.gtk.rounded-bottom-corners.enabled
-  value: true
+  # Disabled for https://github.com/zen-browser/desktop/issues/6302, 
+  # also see https://bugzilla.mozilla.org/show_bug.cgi?id=1979083
+  value: false
   condition: "defined(MOZ_WIDGET_GTK)"
 
 - name: zen.widget.linux.transparency