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/gdm/util.js

@@ -521,7 +521,7 @@ export class ShellUserVerifier extends Signals.EventEmitter {
             this._defaultService = FINGERPRINT_SERVICE_NAME;
 
         if (!this._defaultService) {
-            log("no authentication service is enabled, using password authentication");
+            log('no authentication service is enabled, using password authentication');
             this._defaultService = PASSWORD_SERVICE_NAME;
         }
     }