FIXED assemble error because OpenSearch move strings into core. AS op…

…ensearch-project/OpenSearch#9103 shows.

Signed-off-by: luyuncheng <luyuncheng@bytedance.com>

src/main/java/org/opensearch/knn/index/mapper/MethodFieldMapper.java

@@ -7,7 +7,6 @@
 
 import org.apache.lucene.document.FieldType;
 import org.opensearch.common.Explicit;
-import org.opensearch.common.Strings;
 import org.opensearch.common.xcontent.XContentFactory;
 import org.opensearch.knn.index.KNNMethodContext;
 import org.opensearch.knn.index.util.KNNEngine;
@@ -50,7 +49,7 @@ public class MethodFieldMapper extends KNNVectorFieldMapper {
         try {
             this.fieldType.putAttribute(
                 PARAMETERS,
-                Strings.toString(XContentFactory.jsonBuilder().map(knnEngine.getMethodAsMap(knnMethodContext)))
+                XContentFactory.jsonBuilder().map(knnEngine.getMethodAsMap(knnMethodContext)).toString()
             );
         } catch (IOException ioe) {
             throw new RuntimeException(String.format("Unable to create KNNVectorFieldMapper: %s", ioe));