Fix spec for support faraday 2

[ryogift]

Fixed specs with reference to the upgrade guide to support Faraday 2.0.
Fixed Faraday::Builder#build cannot be used.

https://github.com/lostisland/faraday/blob/main/UPGRADING.md#others

Faraday::Builder#build method is not exposed through Faraday::Connection anymore and does not reset the handlers if called multiple times. This method should be used internally only.

Related to the following issues.
#558
https://github.com/oauth-xx/oauth2/issues/559

[jrochkind]

Hi, I'm looking at backporting to 1-4-stable, and I noted.... while the gemspec allows faraday 2.x, it may not actually be USED in CI, because of the linting dependency on danger ~> 8.4:

oauth2/Gemfile

Line 16 in 7564b81

gem 'danger', '~> 8.4'

8.4.2 is the latest release of danger, and does not allow faraday 2.0. https://rubygems.org/gems/danger/versions/8.4.2

I am confused trying to find the latest CI run here to look in logs for the bundle install output to see if faraday 2.0 is really being tested or not -- so I'm not certain.

Locally, if I manually comment out the danger dependency in Gemfile, I can update to faraday 2.2.0, and bundle exec rspec is green on master at commit da05429.

So tests are passing with faraday 2.x, but I'm not sure that CI is going to run with faraday 2.x, and it requires local changes to Gemfile to run locally with faraday 2.x and bundle exec rspec.

Filed with danger at danger/danger#1349

(And, wow, there is a serious chain of cascading dependency changes necessary to get faraday 2.x actually working).