avoiding NoSuchMethod exception (#6658)

java/com/google/flatbuffers/FlexBuffers.java

@@ -23,6 +23,7 @@ import static com.google.flatbuffers.FlexBuffers.Unsigned.shortToUnsignedInt;
 
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
+import java.nio.Buffer;
 import java.nio.charset.StandardCharsets;
 
 /// @file
@@ -688,7 +689,7 @@ public class FlexBuffers {
          */
         public ByteBuffer data() {
             ByteBuffer dup = ByteBuffer.wrap(bb.data());
-            dup.position(end);
+            ((Buffer) dup).position(end);
             dup.limit(end + size());
             return dup.asReadOnlyBuffer().slice();
         }