CREATE TABLE AS has been preferred over SELECT INTO (outside of ecpg
and PL/pgSQL) for a long time.  There were still a few uses of SELECT
INTO in tests and documentation, some old, some more recent.  This
changes them to CREATE TABLE AS.  Some occurrences in the tests remain
where they are specifically testing SELECT INTO parsing or similar.

Discussion: https://www.postgresql.org/message-id/flat/96dc0df3-e13a-a85d-d045-d6e2c85218da%40enterprisedb.com