Depends on deprecated / unmaintained `node-slackr`

This addon notifies Slack by way of [`node-slackr`](https://github.com/chenka/node-slackr), which has not been updated since mid-2018. The readme indicates it was deprecated at that point, and points to using the [official Slack SDK](https://github.com/slackapi/node-slack-sdk) instead.

`node-slackr` also has a dependency on `request` which has been [fully deprecated](https://github.com/request/request/issues/3142) for some time and has a [known vulnerability](https://github.com/advisories/GHSA-p8p7-x288-28g6) (though I will concede it's unlikely that vulnerability would have an effect on a typical use case here).