For the Mastodon v4.4 release, you need at least Redis v6.2+. Unfortunately if you're running Ubuntu 22.04, the latest distro release of Redis is v6.0 which is too old. So you need the official Redis PPA instead. (Assuming you want to avoid the full Ubuntu update, which I happen to.)
Simply adding the PPA is not enough, because Ubuntu tries to block it in favor of its own ESM (Extended Security Maintenance) version. So it ends up being kind of a pain.
This is a simple bash script you can run (I encourage running every command line-by-line just in case) which successfully