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/misc/introspect.js

@@ -75,7 +75,7 @@ export class IntrospectService {
     _syncRunningApplications() {
         let tracker = Shell.WindowTracker.get_default();
         let apps = this._appSystem.get_running();
-        let seatName = "seat0";
+        let seatName = 'seat0';
         let newRunningApplications = {};
 
         let newActiveApplication = null;