Revoke PUBLIC CREATE from public schema, now owned by pg_database_owner.

Revoke PUBLIC CREATE from public schema, now owned by pg_database_owner.

This switches the default ACL to what the documentation has recommended
since CVE-2018-1058.  Upgrades will carry forward any old ownership and
ACL.  Sites that declined the 2018 recommendation should take a fresh
look.  Recipes for commissioning a new database cluster from scratch may
need to create a schema, grant more privileges, etc.  Out-of-tree test
suites may require such updates.

Reviewed by Peter Eisentraut.

Discussion: https://postgr.es/m/20201031163518.GB4039133@rfd.leadboat.com