Build passes locally with a released version of Postgres 16.0 and still
builds with previous version. Still missing CI changes to pass all the test
suite with PGVERSION=16.

See #1005.

The internal organisation of the header files in Postgres 16 changed in a
way that we can't poke into libpq-int.h anymore, when building on debian,
because of the way the include/common bits are including in libpq (16)
rather than in the postgresql-server-dev-16 parts.

That said, we can stop looking into the internal details of libpq:

  - connection->last_sqlstate allowed an optimisation to conclude that the
    connection is okay early in the polling.

  - connection->auth_req_received allowed to discard authentication error as
    connectivity problems, but we decided in the past that we need to deploy
	a monitor user on the Postgres nodes anyway, to avoid flooding Postgres
	logs with authentication errors.

Allow building for Postgres 16 and Citus 12.1.0.