Bump backoff version for watchtower-plugin

[sr-gi]

The current version of backoff does issue a logging.error message when the backoff strategy is abandoned after reaching the maximum retry count. That log entry does create a BROKEN log entry on c-lightning, which makes test fail if allow_broken_logs is not set.

Furthermore, giving up the backoff should be a warning for us, instead of an error, since we do have a manual retry command to restart the whole process.

Backoff has recently merged a change so the log levels can be set (litl/backoff#99), but it's not yet released. Bump the dependency version once a new release is available.