fix: Subdirectory module names

The module name for the config should be gitlab.com/gitlab-org/api/client-go/config/v2 instead of gitlab.com/gitlab-org/api/client-go/v2/config to properly resolve config/v2.Y.Z versions. The same applies to the graphql_test module. See https://go.dev/ref/mod

In short, the v2/config module path requires a corresponding v2 directory.

Follows !2843 (merged)