Fix not-backwards-compatible pg_upgrade test for prepared transactions.
Fix not-backwards-compatible pg_upgrade test for prepared transactions.

There's no reason for this test to use the undocumented pg_prepared_xact()
function, when it can use the stable API pg_prepared_xacts instead.
Fixes breakage against 8.3, as reported by Justin Arnold.

greenplum-db/gpdb,ovr/postgres-xl,50wu/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,50wu/gpdb,snaga/postgres-xl,adam8157/gpdb,arcivanov/postgres-xl,zeroae/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,arcivanov/postgres-xl,techdragon/Postgres-XL,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,yazun/postgres-xl,lisakowen/gpdb,snaga/postgres-xl,jmcatamney/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,ovr/postgres-xl,ovr/postgres-xl,kmjungersen/PostgresXL,kmjungersen/PostgresXL,jmcatamney/gpdb,tpostgres-projects/tPostgres,ashwinstar/gpdb,50wu/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,50wu/gpdb,tpostgres-projects/tPostgres,adam8157/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,snaga/postgres-xl,xinzweb/gpdb,50wu/gpdb,tpostgres-projects/tPostgres,ashwinstar/gpdb,techdragon/Postgres-XL,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,pavanvd/postgres-xl,postmind-net/postgres-xl,oberstet/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,postmind-net/postgres-xl,lisakowen/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,yazun/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,snaga/postgres-xl,pavanvd/postgres-xl,pavanvd/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,ovr/postgres-xl,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,lisakowen/gpdb,oberstet/postgres-xl,lisakowen/gpdb,kmjungersen/PostgresXL,xinzweb/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,arcivanov/postgres-xl,adam8157/gpdb,postmind-net/postgres-xl,zeroae/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,postmind-net/postgres-xl,oberstet/postgres-xl,adam8157/gpdb,postmind-net/postgres-xl,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,arcivanov/postgres-xl,xinzweb/gpdb,tpostgres-projects/tPostgres,yazun/postgres-xl,yazun/postgres-xl,tpostgres-projects/tPostgres,Postgres-XL/Postgres-XL,greenplum-db/gpdb,snaga/postgres-xl