Make pg_createsubscriber warn if publisher has two-phase commit enabled.

Make pg_createsubscriber warn if publisher has two-phase commit enabled.

pg_createsubscriber currently always sets up logical replication
with two-phase commit disabled.  Improving that is not going to
happen for v17.  In the meantime, document the deficiency, and
adjust pg_createsubscriber so that it will emit a warning if
the source installation has max_prepared_transactions > 0.

Hayato Kuroda (some mods by Amit Kapila and me), per complaint from
Noah Misch

Discussion: https://postgr.es/m/20240623062157.97.nmisch@google.com