Review potentially breaking Slack API fail alerts

[stucka]

github.com/slackapi/slack-github-action introduced potentially breaking changes in v2.0.0.

We're using v1something in etl.yml and likely some other workflows for fail alerts.

I bumped warn-github-flow from v1.25 to v1.27 on the premise that requires no testing.

The Python upgrades in #34 include many other updates, including efforts to upgrade the node backend. Going to a maintained version of the Slack API for fail alerts seems like a sensible add-on.