cleanup: Always use single quotes

We have made good progress with transitioning to the new style,
to the point where we can complete it with a final push.

Start with changing the remaining places that still use double
quotes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

js/ui/windowAttentionHandler.js

@@ -17,7 +17,7 @@ export class WindowAttentionHandler {
 
     _getTitleAndBanner(app, window) {
         let title = app.get_name();
-        let banner = _("“%s” is ready").format(window.get_title());
+        let banner = _('“%s” is ready').format(window.get_title());
         return [title, banner];
     }