Fix requiring of oauth2

The changes in #83 (which were released in v4.1.0) moved the
`OauthClient` class (which uses the `oauth2` gem) from the Rails
applications into the `rpi_auth` gem. At that point the `oauth2` gem was
made a direct dependency of `rpi_auth` and the direct dependency on
`oauth2` was removed from the Rails applications. I _think_ this meant
that the `oauth2` gem was no longer being required automatically. And,
although individual files in the `oauth2` gem were being required from
individual files in `rpi_auth`, this meant that not all of the files in
`oauth2` were being required. In particular, the `lib/oauth2.rb` file
which defines the top-level `OAuth2` module and the `OAuth2.config`
method were no longer being required. This led to exceptions like this
one [1] in `experience-cs` when
`RpiAuth::OauthClient#refresh_credentials` was called:

    NoMethodError: undefined method 'config' for module OAuth2

This commit changes the require statements to just require `oauth2`
which in turn requires all the relevant files [2] in the `oauth2` gem.
The require statement in `spec/rpi_auth/oauth_client_spec.rb` is
unnecessary, because it's automatically loading `RpiAuth::OauthClient`
which in turn loads `oauth2`. This should mean we no longer see
exceptions like the one above.

[1]: https://raspberrypifoundation.sentry.io/issues/6590882516?project=4508953237651456
[2]: https://gitlab.com/oauth-xx/oauth2/-/blob/v2.0.9/lib/oauth2.rb?ref_type=tags#L11-23

Author: floehopper

lib/rpi_auth/controllers/auto_refreshing_token.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'oauth2/error'
+require 'oauth2'
 
 module RpiAuth
   module Controllers

lib/rpi_auth/oauth_client.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
-require 'oauth2/client'
-require 'oauth2/access_token'
+require 'oauth2'
 
 module RpiAuth
   class OauthClient

spec/dummy/spec/requests/refresh_credentials_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-require 'oauth2/error'
+require 'oauth2'
 
 RSpec.describe 'Refreshing the auth token', type: :request do
   include ActiveSupport::Testing::TimeHelpers

spec/rpi_auth/oauth_client_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'spec_helper'
-require 'oauth2'
 
 RSpec.describe RpiAuth::OauthClient do
   include ActiveSupport::Testing::TimeHelpers