`revert-pg-upgrade` fails to downgrade a Geo secondary’s standalone tracking DB’s PG data

After running revert-pg-upgrade on the Geo secondary's standalone tracking DB PG node, the PG data is not downgraded, so the PG server doesn't work:

2020-04-10_00:21:01.27720 FATAL:  database files are incompatible with server
2020-04-10_00:21:01.27722 DETAIL:  The data directory was initialized by PostgreSQL version 11, which is not compatible with this version 10.12.

Further consequences

The DB seems to be in a fairly difficult to recover state since pg-upgrade doesn't work from this point either:

root@mkozono-omnibus4838b-secondary-geo:~# gitlab-ctl pg-upgrade --target-version=11
Checking for an omnibus managed postgresql: OK
Checking if postgresql['version'] is set: OK
Checking if we already upgraded: NOT OK
Checking for a newer version of PostgreSQL to install
Upgrading PostgreSQL to 11.7
Checking if PostgreSQL bin files are symlinked to the expected location: OK
Starting the geo database
Waiting 30 seconds to ensure tasks complete before PostgreSQL upgrade.
See https://docs.gitlab.com/omnibus/settings/database.html#upgrade-packaged-postgresql-server for details
If you do not want to upgrade the PostgreSQL server at this time, enter Ctrl-C and see the documentation for details

Please hit Ctrl-C now if you want to cancel the operation.
Toggling deploy page:cp /opt/gitlab/embedded/service/gitlab-rails/public/deploy.html /opt/gitlab/embedded/service/gitlab-rails/public/index.html
Toggling deploy page: OK
Toggling services:ok: down: crond: 0s, normally up
ok: down: grafana: 1s, normally up
ok: down: logrotate: 0s, normally up
Toggling services: OK
There was an error fetching locale and encoding information from the database
Please ensure the database is running and functional before running pg-upgrade
STDOUT:
STDERR: psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/opt/gitlab/postgresql/.s.PGSQL.5432"?
== Fatal error ==
Please check error logs
== Reverting ==
Symlink correct version of binaries: OK
== Reverted ==
== Reverted to 10.12. Please check output for what went wrong ==
Toggling deploy page:rm -f /opt/gitlab/embedded/service/gitlab-rails/public/index.html
Toggling deploy page: OK
Toggling services:ok: run: crond: (pid 15140) 1s
ok: run: grafana: (pid 15148) 0s
ok: run: logrotate: (pid 15158) 1s
Toggling services: OK

Losing the tracking DB data isn't the absolute worst, since you can wipe it and resync everything, but for large instances, that could take a long time, and if this Geo secondary is used for DR purposes, you are unprotected during that time. Setting gitlab3713902 for that reason, though this is IMO a low priority gitlab3713902.