pgsql: Change default of password_encryption to scram-sha-256

Change default of password_encryption to scram-sha-256

Also, the legacy values on/true/yes/1 for password_encryption that
mapped to md5 are removed. The only valid values are now
scram-sha-256 and md5.

Reviewed-by: Jonathan S. Katz <jkatz(at)postgresql(dot)org>
Discussion: https://www.postgresql.org/message-id/flat/d5b0ad33-7d94-bdd1-caac-43a1c782cab2%402ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c7eab0e97e6cf1d0c136c22269c10ae11ba874c4

Modified Files
--------------
doc/src/sgml/config.sgml | 10 +++++-----
src/backend/commands/user.c | 2 +-
src/backend/utils/misc/guc.c | 11 +----------
src/backend/utils/misc/postgresql.conf.sample | 2 +-
src/bin/initdb/initdb.c | 21 +++++++++++----------
src/test/regress/expected/password.out | 5 +++--
src/test/regress/sql/password.sql | 3 +--
7 files changed, 23 insertions(+), 31 deletions(-)