chore: update PaymentRequest to use String productCode instead of UUID

Author: otaviojava

src/main/java/com/acme/payment/PaymentRequest.java

@@ -1,9 +1,8 @@
 package com.acme.payment;
 
 import java.math.BigDecimal;
-import java.util.UUID;
 
-public record PaymentRequest(UUID productCode,
+public record PaymentRequest(String productCode,
                              String productName,
                              int quantity,
                              BigDecimal unitPrice) {