Handle currency data on Java 21 (#138)

* Adjust code so the symbol for currency XXX remains as XXX

src/main/java/org/joda/money/CurrencyUnit.java

@@ -577,6 +577,10 @@ public boolean isPseudoCurrency() {
      * @return the JDK currency instance, never null
      */
     public String getSymbol() {
+        // Java 21 currency data uses a symbol, we want to retain this as XXX
+        if ("XXX".equals(code)) {
+            return code;
+        }
         try {
             return Currency.getInstance(code).getSymbol();
         } catch (IllegalArgumentException ex) {
@@ -597,6 +601,10 @@ public String getSymbol() {
      */
     public String getSymbol(Locale locale) {
         MoneyUtils.checkNotNull(locale, "Locale must not be null");
+        // Java 21 currency data uses a symbol, we want to retain this as XXX
+        if ("XXX".equals(code)) {
+            return code;
+        }
         try {
             return Currency.getInstance(code).getSymbol(locale);
         } catch (IllegalArgumentException ex) {

src/test/java/org/joda/money/TestCurrencyUnit.java

@@ -712,6 +712,8 @@ void test_getSymbol_Locale_XXX() {
             Locale.setDefault(Locale.UK);
             CurrencyUnit test = CurrencyUnit.of("XXX");
             assertThat(test.getSymbol(Locale.FRANCE)).isEqualTo("XXX");
+            test = CurrencyUnit.of("XXX");
+            assertThat(test.getSymbol(Locale.US)).isEqualTo("XXX");
         } finally {
             Locale.setDefault(loc);
         }