Downcase aliased OAuth2 callback providers

Users may specify an OAuth2 callback with a custom name, such as AWSCognito, but Rails will reject this with the following message:

import/AWSCognito' is not a supported controller name. This can
lead to potential routing problems. See
http://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use

To avoid these errors, we can just downcase all the provider names. Note that this will make it impossible to specify a duplicate name with different cases.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57156