Fix unnamed variable errors for JDK 21 compatibility

- Replace unnamed variables (_) with named variables (ignored)
- Fixes compilation errors on JDK 21 which doesn't support unnamed variables
- Updated in JsonNumberImpl.java, JsonObject.java, and Json.java

Author: simbo1905

src/main/java/jdk/sandbox/internal/util/json/JsonNumberImpl.java

@@ -80,7 +80,7 @@ public Number toNumber() {
             } else {
                 try {
                     return Long.parseLong(str);
-                } catch (NumberFormatException _) {
+                } catch (NumberFormatException ignored) {
                     return new BigInteger(str);
                 }
             }

src/main/java/jdk/sandbox/java/util/json/Json.java

@@ -257,7 +257,7 @@ public static Object toUntyped(JsonValue src) {
                     .map(Json::toUntyped)
                     .toList();
             case JsonBoolean jb -> jb.value();
-            case JsonNull _ -> null;
+            case JsonNull ignored -> null;
             case JsonNumber n -> n.toNumber();
             case JsonString js -> js.value();
         };

src/main/java/jdk/sandbox/java/util/json/JsonObject.java

@@ -67,7 +67,7 @@ static JsonObject of(Map<String, ? extends JsonValue> map) {
                 .stream()
                 .collect(Collectors.toMap(
                         e -> Objects.requireNonNull(e.getKey()), Map.Entry::getValue, // Implicit NPE on val
-                        (_, v) -> v, LinkedHashMap::new)));
+                        (ignored, v) -> v, LinkedHashMap::new)));
     }
 
     /**