event-editor/event-editor-dialog: Port to AdwPreferencesPage

Our use case is already handled by the `AdwPreferencesPage:banner`
property, which allows us to port the dialog to AdwPreferencesPage.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/781>

src/gui/event-editor/gcal-event-editor-dialog.blp

@@ -31,16 +31,11 @@ template $GcalEventEditorDialog: Adw.Dialog {
           }
         }
 
-        content: Box {
-          orientation: vertical;
-
-          Adw.Banner {
+        content: Adw.PreferencesPage {
+          banner: Adw.Banner {
             title: _("This event is in a read-only calendar. It cannot be edited or deleted.");
             revealed: bind template.writable inverted;
-          }
-
-          Adw.PreferencesPage {
-            vexpand: true;
+          };
 
           Adw.PreferencesGroup {
             $GcalSummarySection summary_section {
@@ -92,7 +87,6 @@ template $GcalEventEditorDialog: Adw.Dialog {
               ]
             }
           }
-          }
         };
       }
     }