Changing the icon to 'system-log-out-symbolic' has no visual
change in a default GNOME setup since both 'system-log-out-symbolic'
and 'application-exit-symbolic' are the same in adwaita-icon-theme
(at the time of writing), however, other icon themes differentiate
between the two icons so pointing to the appropriate icon name
is the right thing to do.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2751
(cherry picked from commit 7d60f418e7)