Fix cross-version upgrade tests after f0827b443.

Removing the get_columns_length() function from regress.so
means we have to drop it when testing upgrades from versions
that had it.  Per buildfarm.

Discussion: https://postgr.es/m/2520881.1709159002@sss.pgh.pa.us

src/test/perl/PostgreSQL/Test/AdjustUpgrade.pm

@@ -106,6 +106,13 @@ sub adjust_database_contents
 			'drop extension if exists test_ext7');
 	}
 
+	# we removed this test-support function in v17
+	if ($old_version >= 15 && $old_version < 17)
+	{
+		_add_st($result, 'regression',
+			'drop function get_columns_length(oid[])');
+	}
+
 	# stuff not supported from release 16
 	if ($old_version >= 12 && $old_version < 16)
 	{