feat: fold purchase and promo voucher fields into one code field

src/Form/Service/ParticipantFieldHandlerRegistry.php

@@ -263,6 +263,9 @@ class ParticipantFieldHandlerRegistry
             'dropOff',
             'parking',
             'licensePlate',
+            // Cleared by the voucher handler once applied or removed; must not be bound again
+            'voucherCode',
+            'removeVoucherCode',
         ];
 
         foreach ($serviceFields as $fieldName) {