Set explicit visibility on class constants

src/MDAppointmentAttendanceModeSet.php

@@ -12,13 +12,13 @@
  */
 final class MDAppointmentAttendanceModeSet extends MDValueSet {
 
-    const MODES = [
+    public const MODES = [
         'offline',
         'online',
         'mixed',
     ];
 
-    const MODES_TO_SCHEMA_ORG = [
+    public const MODES_TO_SCHEMA_ORG = [
         'offline' => 'https://schema.org/OfflineEventAttendanceMode',
         'online' => 'https://schema.org/OnlineEventAttendanceMode',
         'mixed' => 'https://schema.org/MixedEventAttendanceMode',