Warn upon successful MD5 password authentication.

Warn upon successful MD5 password authentication.

This uses the "connection warning" infrastructure introduced by
commit 1d92e0c2cc to emit a WARNING when an MD5 password is used to
authenticate.  MD5 password support was marked as deprecated in
v18 and will be removed in a future release of Postgres.  These
warnings are on by default but can be turned off via the existing
md5_password_warnings parameter.

Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Reviewed-by: Xiangyu Liang <liangxiangyu_2013@163.com>
Discussion: https://postgr.es/m/aYzeAYEbodkkg5e-%40nathan