Fix spec for support faraday 2
Created by: 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. https://github.com/oauth-xx/oauth2/pull/558 https://github.com/oauth-xx/oauth2/issues/559