Deprecate identifier collection approach.

[dereuromark]

Refs #710

The config has to be only slightly adjusted to be "safer".
Seems BC and easy to switch up.

[dereuromark]

Added also a test for a direct IdentifierCollection passed:
f2c713e
Or is that also deprecated? I guess it can be useful in some rare cases.

[ADmad]

There's AuthenticationService::getIdentificationProvider() which returns the identifier instance which successfully returned the identity. So that method needs to be updated too to get the identifier instance from the successful authenticator (if the IdentifierCollection::getIdentificationProvider() instance for the AuthenticationServices returns null).

[dereuromark]

Good point.
Done :)

[dereuromark]

Are we good to continue here?

[markstory]

Thanks for adding that configuration test. There are a few more errors in the docs.

[dereuromark]

We should definitly squash merge this one 🗡️

[dereuromark]

Is this going into 3.x or 3.next?