feat: publish release notes to Slack

The GitHub repo changelog modifier wasn't working as desired because it
was introducing a new changelog file each time, replacing all the old
release notes. It's a whole lot easier for us to just publish these
individual version release notes straight to Slack, where the history of
all previous releases is kept.

BREAKING CHANGE: Must be run with CHANGELOG_CHANNEL_URL set on the
environment (as a Slack Incoming Webhook URL)