js: Adapt for ::child-{added,removed}

These were previously known as ::actor-{added,removed}

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3014>

js/ui/calendar.js

@@ -1005,8 +1005,8 @@ class CalendarMessageList extends St.Widget {
             y_align: Clutter.ActorAlign.START,
         });
         this._sectionList.connectObject(
-            'actor-added', this._sync.bind(this),
-            'actor-removed', this._sync.bind(this),
+            'child-added', this._sync.bind(this),
+            'child-removed', this._sync.bind(this),
             this);
         this._scrollView.child = this._sectionList;