Replace the hand-rolled entrypoint.postgres.sh.tmpl (which apt-installed
the old PG binaries and ran initdb + pg_upgrade --link by hand) with the
pgautoupgrade/pgautoupgrade:18-alpine image, matching the other recipes.
PGDATA is pinned to the legacy /var/lib/postgresql/data so the existing
cluster on the volume is upgraded in place rather than re-initialised.
Drops the db_entrypoint config and DB_ENTRYPOINT_VERSION.
