Avoid to use the config alias while supporting Doorkeeper 5.2

Doorkeeper.configuration has been aliased as Doorkeeper.config from Doorkeeper v5.3.0. This alias has been used in UserInfoController from doorkeeper-openid_connect v1.7.3.

However the latest doorkeeper-openid_connect is still supporting Doorkeeper v5.2.* and Doorkeeper.config does not work in those versions. To fix this, we should use the original method name while the support of Doorkeeper v5.2.* continues.