Docs only refers to Postgres Flex by default

[guillaumervls]

Postgres Flex is what we deploy by default now. We should remove or mark as legacy mentions to our old Postgres setup.

We should maybe also update our PG blog post.

[redjonzaci]

There are 4 mentions in the blog post, but the link Running Locally (for Fly Employees) is broken.

[andie787]

I would say this is done done now