[WCM] [Cookbook] [Security] Change the interface which needs to be implemented by a user repository

[mtrojanowski]

The symfony ticket: symfony/symfony#12733

If the code is merged, in this artice http://symfony.com/doc/current/cookbook/security/entity_provider.html#authenticating-someone-with-a-custom-entity-provider the interface used by the repository should be changed to the new interface and the supportsClass function can be deleted from the implementation.

[mtrojanowski]

This is now connected to the PR #15947, which introduces slightly different changes. Nevertheless it will need a fix in the docs if the PR is accepted so please keep this issue open.

[wouterj]

Will be fixed by #5993