Update oauth.rb for compatibility with rack v3.1+

[JamesChevalier]

This line causes logins to fail in rack v3.1+ https://github.com/omniauth/omniauth-oauth/blob/master/lib/omniauth/strategies/oauth.rb#L51

It must be changed from opts[:oauth_verifier] = request["oauth_verifier"] to opts[:oauth_verifier] = request.params["oauth_verifier"]

Due to my Rails app's state, I was only able to test request.params["oauth_verifier"] going back to rack v2.2.4 where it continued to be functional.

Let me know if you'd prefer something like Gem.loaded_specs["rack"].version >= Gem::Version.new("3.0") to conditionally switch to the updated syntax.