Update pg_hba.conf example to reflect MD5 deprecation

In the wake of commit db6a4a985, remove most use of 'md5' from the
example configuration file. The only remainder is an example exception
for a client that doesn't support SCRAM.

Author: Mikael Gustavsson <mikael.gustavsson@smhi.se>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/176595607507.978865.11597773194269211255@wrigleys.postgresql.org
Discussion: https://postgr.es/m/4ed268473fdb4cf9b0eced6c8019d353@smhi.se
Backpatch-through: 18

doc/src/sgml/client-auth.sgml

@@ -889,16 +889,16 @@ host    all             all             192.168.0.0/16          ident map=omicro
 # list of names of administrators.  Passwords are required in all cases.
 #
 # TYPE  DATABASE        USER            ADDRESS                 METHOD
-local   sameuser        all                                     md5
-local   all             /^.*helpdesk$                           md5
-local   all             @admins                                 md5
-local   all             +support                                md5
+local   sameuser        all                                     scram-sha-256
+local   all             /^.*helpdesk$                           scram-sha-256
+local   all             @admins                                 scram-sha-256
+local   all             +support                                scram-sha-256
 
 # The last two lines above can be combined into a single line:
-local   all             @admins,+support                        md5
+local   all             @admins,+support                        scram-sha-256
 
 # The database column can also use lists and file names:
-local   db1,db2,@demodbs  all                                   md5
+local   db1,db2,@demodbs  all                                   scram-sha-256
 </programlisting>
    </example>
  </sect1>