Make Oktakit::Response::RaiseError#on_complete public

A respond_to? guard clause in Faraday::Middleware prevents this
callback from ever executing when the method is private.

See https://github.com/lostisland/faraday/blob/main/lib/faraday/middleware.rb#L18

lib/oktakit/response/raise_error.rb

@@ -7,8 +7,6 @@ module Response
     # This class raises an Oktakit-flavored exception based
     # HTTP status codes returned by the API
     class RaiseError < Faraday::Response::Middleware
-      private
-
       def on_complete(response)
         if (error = Oktakit::Error.from_response(response))
           raise error