gui: Replace dim-label with dimmed style class

`dim-label` is deprecated.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/624>

src/gui/views/gcal-week-hour-bar.c

@@ -106,7 +106,7 @@ gcal_week_hour_bar_init (GcalWeekHourBar *self)
       GtkWidget *label = gtk_label_new ("");
 
       gtk_widget_add_css_class (label, "line");
-      gtk_widget_add_css_class (label, "dim-label");
+      gtk_widget_add_css_class (label, "dimmed");
       gtk_widget_set_vexpand (label, TRUE);
       gtk_label_set_yalign (GTK_LABEL (label), 0.0);
       gtk_box_append (GTK_BOX (self), label);