Looks like we will need to rebuild our (single) data container soon to get rid of this error in Discourse. Or, I will just build Discourse for scratch and restore the data and switch over when it is done to minimize down time.
Rebuilding our data container did not work as per the meta instructions (above); but I managed to improvise and 30 minutes later, it's running again and all is good.
As I understand meta from the past, they consider multi-containers solutions "not supported" so it is not a surprise that the instructions above did not work for us. I knew going into the data container rebuild that it would not go smoothly per the meta-talk on this bug.
The good news, is that bug is quashed, I assume, with the PG upgrade and new data container.
.... just because I was already working on the DB and seems like a good housekeeping thing to do.
The site does seem a little faster after housekeeping; but that could all be in my imagination. I notice our cars and motorcycles always seem to run better after we wash them