Update pg_hba.conf example to reflect MD5 deprecation

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