systemActions: Use 'system-log-out-symbolic' for logout

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

js/misc/systemActions.js

@@ -107,7 +107,7 @@ const SystemActions = GObject.registerClass({
         this._actions.set(LOGOUT_ACTION_ID, {
             // Translators: The name of the logout action in search
             name: C_("search-result", "Log Out"),
-            iconName: 'application-exit-symbolic',
+            iconName: 'system-log-out-symbolic',
             // Translators: A list of keywords that match the logout action, separated by semicolons
             keywords: tokenizeKeywords(_('logout;log out;sign off')),
             available: false,