Return Gateway Timeout instead of Request Timeout for database timeout

Closes #14945
(grafted from ae46da1d0ab434dda58b54070e1faa2a72974935)

--HG--
branch : 8.0

trytond/trytond/protocols/dispatcher.py

@@ -251,7 +251,7 @@ def _dispatch(request, pool, *args, **kwargs):
             except backend.DatabaseTimeoutError:
                 logger.warning(
                     log_message, *log_args, duration(), exc_info=True)
-                abort(HTTPStatus.REQUEST_TIMEOUT)
+                abort(HTTPStatus.GATEWAY_TIMEOUT)
             except backend.DatabaseOperationalError:
                 if count < retry and not rpc.readonly:
                     transaction.rollback()