Fix deprecated index_together for django 5.1

Author: dishn

clickhouse_backend/backend/schema.py

@@ -213,10 +213,14 @@ def _model_indexes_sql(self, model):
         )
         if (
             any(field.db_index for field in model._meta.local_fields)
-            or model._meta.index_together
+            or any(len(index.fields) > 1 for index in model._meta.indexes)
         ):
             warnings.warn(msg)
 
+        # index_together is deprecated in django 5.1
+        if hasattr(model._meta, "index_together") and model._meta.index_together:
+            warnings.warn(msg)
+
         for index in model._meta.indexes:
             output.append(index.create_sql(model, self))