Move all codegen templates to codegen-templates (#1899)

## Motivation

We can't have the Go codegen templates inside the `go/` directory
because they get interpreted as regular source files then. So we have
them in a separate directory. It's a bit weird that we only do this for
Go. We can just move all of them for consistency.

## Solution

Rename `openapi-templates` => `codegen-templates` for a little extra
clarity (also means this sorts before all the SDKs in file listings) and
move everything there.

Author: svix-jplatte