Don't use system dependent charset

Fixes quarkusio#23991

(cherry picked from commit ad36396)

extensions/vertx-http/runtime/src/main/java/io/quarkus/vertx/http/runtime/devmode/FileSystemStaticHandler.java

@@ -2,7 +2,7 @@
 
 import java.io.Closeable;
 import java.io.IOException;
-import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.FileSystem;
 import java.nio.file.FileSystems;
 import java.nio.file.Files;
@@ -31,7 +31,7 @@
  */
 public class FileSystemStaticHandler implements Handler<RoutingContext>, Closeable {
 
-    private static final String DEFAULT_CONTENT_ENCODING = Charset.defaultCharset().name();
+    private static final String DEFAULT_CONTENT_ENCODING = StandardCharsets.UTF_8.name();
 
     private static final ReentrantLock ROOT_CREATION_LOCK = new ReentrantLock();