The problem was caused because I started an upgrade and then proceeded to walk away from my computer and do Christmas-type things. It was lovely, but I did have a broken Discourse installation and nobody could read what I just wrote.
I also suspect my use of Oracle’s slow free tier didn’t help. As in, the install probably timed out and left things in an inconsistent state. Fortunately, this isn’t a mission-critical site.
Since the server was so badly broken I couldn’t use the admin page, I decided to update Discourse via the command line. I also update the software on the instance. I also updated the OS to Ubuntu 22.04.1 LTS (Jammy Jellyfish). It took a while, but why worry about downtime when I was already down?