Even on Debian 12 (bookwork) we need to use "jessie" in the source list file.

- https://packagecloud.io/slacktechnologies/slack
- https://packagecloud.io/slacktechnologies/slack/install#manual-deb
- Changelog: https://slack.com/intl/en-gb/release-notes/linux

Debian 13: Repository works on 2026-10-02.