Replace Collections.singletonList() with List.of()

Use the more modern and concise List.of() factory method instead of
Collections.singletonList() for creating immutable single-element lists.
This is the preferred approach in Java 9+.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: oschwald

src/main/java/com/maxmind/minfraud/WebServiceClient.java

@@ -103,7 +103,7 @@ public static final class Builder {
         Duration connectTimeout;
         Duration requestTimeout;
 
-        List<String> locales = Collections.singletonList("en");
+        List<String> locales = List.of("en");
         private ProxySelector proxy;
         private HttpClient httpClient;
 

src/test/java/com/maxmind/minfraud/response/AbstractOutputTest.java

@@ -11,7 +11,7 @@
 import com.fasterxml.jackson.databind.util.StdDateFormat;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 import java.io.IOException;
-import java.util.Collections;
+import java.util.List;
 
 public abstract class AbstractOutputTest {
 
@@ -27,7 +27,7 @@ <T> T deserialize(Class<T> cls, String json) throws IOException {
             .serializationInclusion(JsonInclude.Include.NON_EMPTY)
             .build();
         var inject = new Std().addValue(
-            "locales", Collections.singletonList("en"));
+            "locales", List.of("en"));
         return mapper.readerFor(cls).with(inject).readValue(json);
     }
 }