utils: Align GcalWeekDay enum with ISO 8601

Start the enum from Monday-Sunday (1-7). Adapt code that references the
`is_workday()` utility method, which in turn uses the GcalWeekDay enum
to combine pairs of days by bitwise or-ing.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/822>

src/gui/common/gcal-date-chooser-day.c

@@ -199,7 +199,7 @@ gcal_date_chooser_day_set_date (GcalDateChooserDay *self,
 
   now = g_date_time_new_now (g_date_time_get_timezone (date));
   today = gcal_date_time_compare_date (date, now) == 0;
-  weekday = g_date_time_get_day_of_week (date) % GCAL_N_WEEKDAYS;
+  weekday = g_date_time_get_day_of_week (date);
 
   if (G_UNLIKELY (today))
     gtk_widget_add_css_class (widget, "today");