Demote Failed to query device keys for some users warning to level=debug

Many of these warnings are due to dead servers and are quite annoying when they fill up the logs.

keyserver/internal/device_list_update.go

@@ -424,7 +424,7 @@ func (u *DeviceListUpdater) processServer(serverName gomatrixserverlib.ServerNam
 			"succeeded": successCount,
 			"failed":    len(userIDs) - successCount,
 			"wait_time": waitTime,
-		}).Warn("Failed to query device keys for some users")
+		}).Debug("Failed to query device keys for some users")
 	}
 	return waitTime, !allUsersSucceeded
 }