The original value 12 was set based on RFC 5802 for SCRAM-SHA-1, but RFC
7677 for SCRAM-SHA-256 uses 16, so use that.  (This does not affect the
validity of already stored verifiers.)

Discussion: https://www.postgresql.org/message-id/flat/12cc9297-7e05-932f-d863-765e5626ead4%402ndquadrant.com