Add backward compatibility for core__sql_alchemy_conn__cmd (#23441)

(cherry picked from commit 0cdd401)

airflow/configuration.py

@@ -140,6 +140,8 @@ class AirflowConfigParser(ConfigParser):
         ('atlas', 'password'),
         ('smtp', 'smtp_password'),
         ('webserver', 'secret_key'),
+        # The following options are deprecated
+        ('core', 'sql_alchemy_conn'),
     }
 
     # A mapping of (new section, new option) -> (old section, old option, since_version).