fix: system tests failure from test_upload_and_deploy_xgboost_model (#1149)

Fixes #1138 🦕

Author: morgandu

google/cloud/aiplatform/models.py

@@ -2775,7 +2775,7 @@ def upload_xgboost_model_file(
                 Also if model directory does not contain a supported model file.
         """
         if not display_name:
-            display_name = cls.__class__.__generate_display_name("XGBoost model")
+            display_name = cls._generate_display_name("XGBoost model")
 
         XGBOOST_SUPPORTED_MODEL_FILE_EXTENSIONS = [
             ".pkl",