Switch to IRegistrationContext::registerAlternativeLogin

[julien-nc]

As mentioned in https://github.com/nextcloud/user_oidc/blob/4db5bc0/lib/AppInfo/Application.php#L113
OC_App::registerLogIn is deprecated since NC 20.

The new way to register a login method (leading to an extra button in the login form) is to call IRegistrationContext::registerAlternativeLogin in the app's Application::register.

Problems:

• Getting the db connection (\OC::$server->get(ProviderMapper::class); might work)
• We can only register classes with the new system. How are we going to register the same class multiple times but make it point to different oidc providers?

@juliushaertl Not very urgent but at some point the deprecated OC_App::registerLogIn will be gone.

[juliushaertl]

We can only register classes with the new system. How are we going to register the same class multiple times but make it point to different oidc providers?

Yeah that one is tricky, I don't see a good way of migrating without adjustments to the server API for that.