So somewhere you changed the postgres port to 50432 instead of 5432? (OR maybe the migration code does that and I’ve never noticed).
Maybe switch to the postgres13 template as suggested in the OP? And if that works, what I would do is move to a new server and avoid the whole transition-to-new-postgres-version issue and just restore the database to the new server.
I didn’t change it, and that socket does exist so… 
I did that for now, but I assume that’s a bad idea long-term
I’ll let the one who manages the server know ig
So maybe it does run two postgres to make the move.
True, but I’m relieved that your back up!
I don’t have enough space to perform this upgrade on my Discourse partition. However, I have plenty of space on another drive. Is there a way to use this for the temporary storage?
It’s probably time for an OS upgrade anyway, and moving to a fresh VM is much easier, requires little downtime, and if something goes wrong, you still have a working server.
If you’ve got a separate data container, you can try moving all of /var/discourse/shared/data to the other partition and adjusting the volumes in your YML accordingly.
And if you don’t have a separate container, you can do something like that, it’s just more complicated.