[SPARK-56607][PYTHON][FOLLOWUP] Use pyspark.sql.DataFrame to support connect-only

### What changes were proposed in this pull request?

Use `pyspark.sql.DataFrame`, not the classic one, in `mlutils.py`.

### Why are the changes needed?

We have connect only CI which does not even have class DataFrame. This util should work with connect DataFrame too.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

`test_pipeline` and `test_parity_pipeline` passed locally.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #55630 from gaogaotiantian/fix-mlutils.

Authored-by: Tian Gao <gaogaotiantian@hotmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

Author: gaogaotiantian

python/pyspark/testing/mlutils.py

@@ -25,8 +25,7 @@
 from pyspark.ml.classification import Classifier, ClassificationModel
 from pyspark.ml.util import DefaultParamsReadable, DefaultParamsWritable
 from pyspark.ml.wrapper import _java2py
-from pyspark.sql import SparkSession
-from pyspark.sql.classic.dataframe import DataFrame
+from pyspark.sql import DataFrame, SparkSession
 from pyspark.sql.types import DoubleType
 from pyspark.testing.utils import ReusedPySparkTestCase as PySparkTestCase
 
@@ -100,6 +99,11 @@ def tearDownClass(cls):
 
 
 class MockDataset(DataFrame):
+    def __new__(cls, *args, **kwargs):
+        # DataFrame by default creates classic DataFrame, we need this to
+        # overwrite the default behavior.
+        return object.__new__(cls)
+
     def __init__(self):
         self.index = 0