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

Because in the 18 cycle we turned checksums on by default with commit
04bec89, 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

Author: alvherre

src/bin/pg_upgrade/t/002_pg_upgrade.pl

@@ -180,6 +180,10 @@ sub filter_dump
 	}
 }
 
+# Since checksums are now enabled by default, and weren't before 18,
+# pass '-k' to initdb on old versions so that upgrades work.
+push @initdb_params, '-k' if $oldnode->pg_version < 18;
+
 $node_params{extra} = \@initdb_params;
 $oldnode->init(%node_params);
 $oldnode->start;