In pg_upgrade cross-version test, handle postfix operators.
Commit
1ed6b895634ce0dc5fd4bd040e87252b32182cba eliminated support for
them, so drop them from regression databases before upgrading. This is
necessary but not sufficient for testing v13 -> v14 upgrades.
Discussion: https://postgr.es/m/449144.
1600439950@sss.pgh.pa.us