Aha - my file:
/var/discourse/shared/standalone/postgres_data_old/PG_VERSION
has the value “10”.
So the above PostgreSQL 13 link, to the extent that it applies to a 12 ==> 13 upgrade, is NOT necessarily relevant, so far as I know (which is not far at all.)
I am having a really bad feeling about this … like trying to fix live military munitions, in a dark cave, without instructions, untrained, and not even being sure what kind of munitions I’m working on.