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 <mbanck@gmx.net>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Backpatch-through: 18

src/backend/utils/misc/postgresql.conf.sample

@@ -94,9 +94,9 @@
 # - Authentication -
 
 #authentication_timeout = 1min		# 1s-600s
-#password_encryption = scram-sha-256	# scram-sha-256 or md5
+#password_encryption = scram-sha-256	# scram-sha-256 or (deprecated) md5
 #scram_iterations = 4096
-#md5_password_warnings = on
+#md5_password_warnings = on		# display md5 deprecation warnings?
 #oauth_validator_libraries = ''	# comma-separated list of trusted validator modules
 
 # GSSAPI using Kerberos