Warn on mini scheduler failures instead of debug (#39760)

Mini scheduler issues are rare and hard to debug. Chances are the problem won't be reproducible after enabling debug logging.

(cherry picked from commit 38e003c)

airflow/models/taskinstance.py

@@ -3506,7 +3506,7 @@ def _schedule_downstream_tasks(
 
         except OperationalError as e:
             # Any kind of DB error here is _non fatal_ as this block is just an optimisation.
-            cls.logger().debug(
+            cls.logger().warning(
                 "Skipping mini scheduling run due to exception: %s",
                 e.statement,
                 exc_info=True,