Replace serial by generated as identity

[steve-chavez]

Tutorial 0 and tutorial 1 make use of serial and grant usage on sequence.

We should update this to identity columns since users keep hitting on this issue: https://github.com/PostgREST/postgrest/issues/251.

This also follows pg guidelines: https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_serial