Mention md5 deprecation in postgresql.conf.sample

Mention md5 deprecation in postgresql.conf.sample

PostgreSQL 18 deprecated password_encryption='md5', but the
comments for this GUC in the sample configuration file did
not mention the deprecation.  Update comments with a notice
to make as many users as possible aware of it.  Also add a
comment to the related md5_password_warnings GUC while there.

Author: Michael Banck <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Nathan Bossart <[email protected]>
Reviewed-by: Robert Treat <[email protected]>
Backpatch-through: 18