migrate away from JFrog’s Bintray for Debian package distribution

We currently host our auto-built Debian packages on bintray, which JFrog is canceling in May: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

In terms of alternatives that offer free plans for open source projects, I can see:

| URL | Twitter | Pros/Cons |
|--------|----------|----------------|
| https://www.getbalto.com/ | https://twitter.com/BaltoRepo | seems small; not sure how long it will be around (since 2019) |
| https://packagecloud.io/ | https://twitter.com/packagecloudio | (since 2014) |
| launchpad | - | - |

Let me know if anyone knows of any others we should consider.

TODO list:
- [x] update travis configuration to push to the new provider (might also affect work done in #4095)
- [x] update https://i3wm.org/docs/repositories.html
- [x] send announcement: https://freelists.org/post/i3-announce/i3-autobuild-repositories-moved-from-bintray-to-balto
- [ ] implement a cleanup step for the provider we stick with