Fix xxl-job latest dep test (#14515)

Author: laurit

instrumentation/xxl-job/xxl-job-common/testing/src/main/java/io/opentelemetry/instrumentation/xxljob/ReflectiveMethodsFactory.java

@@ -21,7 +21,9 @@ public void initMethod() {}
     public void destroyMethod() {}
 
     public ReturnT<String> echo(String param) {
-      return new ReturnT<>("echo: " + param);
+      ReturnT<String> result = new ReturnT<>();
+      result.setContent("echo: " + param);
+      return result;
     }
   }