pgsql: 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(at)proxel(dot)se>
Reviewed-by: Xiangyu Liang <liangxiangyu_2013(at)163(dot)com>
Discussion: https://postgr.es/m/aYzeAYEbodkkg5e-%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bc60ee86066520ee0f86abd3d47bb8ab9757842f

Modified Files
--------------
doc/src/sgml/config.sgml | 3 ++-
src/backend/libpq/crypt.c | 17 +++++++++++++++++
src/test/authentication/t/001_password.pl | 2 ++
3 files changed, 21 insertions(+), 1 deletion(-)