feat: remove Alt-F5 from GNOME unmaximize keybinding

The exposed GNOME keybinding for GNOME is '<Super>Down', and it's all a
user sees in the GNOME Settings. However, '<Alt>F5' is also there as a
hidden default. This is confusing enough, but the real trouble is this
shortcut conflicts with VSCode's default "next diff" shortcut. As this
hidden shortcut seems somewhat deprecated, I'm removing it here.

system_files/silverblue/usr/share/glib-2.0/schemas/zz1-bos-modifications.gschema.override

@@ -3,6 +3,9 @@ font-name="Inter 10"
 document-font-name="Inter 10"
 monospace-font-name="JetBrains Mono 11"
 
+[org.gnome.desktop.wm.keybindings]
+unmaximize=['<Super>Down']
+
 [org.gnome.desktop.wm.preferences]
 resize-with-right-button=true
 titlebar-font="Inter Bold 10"