This is a rework of 7021d3b176, where we relied on forcing
max_logical_replication_workers to 0 in the postgres command. This
commit now prevents logical replication launchers to start using -b and
a backend-side check based on IsBinaryUpgrade, effective when upgrading
from 17 and newer versions.
This commit improves the comments explaining why this restriction is
necessary.
This discussion was on hold until we were sure how to add support for
subscribers in pg_upgrade, something now done thanks to 9a17be1e24.
Reviewed-by: Álvaro Herrera, Amit Kapila, Tom Lane
Discussion: https://postgr.es/m/ZU2TeVkUg5qEi7Oy@paquier.xyz
08c3ad27eb