Update .gitmodules - opauth submodule

Because the repo moved from https://github.com/uzyn/opauth to https://github.com/opauth/opauth
Maybe its better to update the submodule in this git repo - pointing to the right opauth repo?
Took me a while to notice the `lib` folder is empty.

Updated submodule Vendor/Opauth to match new repo opauth/opauth