Revert XactLockTableWait context setup in conditional multixact wait

There's no point in setting up a context error callback when doing
conditional lock acquisition, because we never actually wait and so the
able wouldn't be able to see it.

Backpatch to 9.4, where this was added.
upgrade docs:  highlight pg_upgrade, warn about globals preservation

Also, remove OID preservation mention, mention non-text dump formats

Backpatch through 9.4