fix(config): return a client-go/v3 client from config.NewClient()

The config module's go.mod was never bumped when it was extracted alongside the v3 release, so config.NewClient() and config.NewClientForContext() still constructed a gitlab.com/gitlab-org/api/client-go/v2 client while the rest of the library moved to v3. Update the config module to depend on and import client-go/v3, and correct the v2-oriented paths/examples in the README and v3 migration guide.

This is technically a breaking change to the config package's return type, but it is a bug fix for behavior that was never intentional.