Fix pg_upgrade's cross-version tests when old < 18

Fix pg_upgrade's cross-version tests when old < 18

Because in the 18 cycle we turned checksums on by default with commit
04bec894a04c, and pg_upgrade fails if the setting doesn't match in old
and new clusters, the built-in cross-version pg_upgrade test is failing
if the old version is older than 18.  Fix the script so that it creates
the old cluster with checksums enabled (-k) in cross-version scenarios.

This went unnoticed because the buildfarm doesn't use the same test code
for cross-version testing.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/202411071838.7fgkb7uvavvz@alvherre.pgsql