Symfony 5.1 compatibility

Created by: drupol

In Symfony 5.1, the security and especially the Guard system has been revamped.

Read:

• https://symfony.com/blog/new-in-symfony-5-1-updated-security-system
• http://wouterj.nl/2020/04/authenticators-new-symfony-security

This issue is to keep track of the evolution and compatibility of this package with Symfony 5.1.

I had a conversation with @wouterj on Slack and this is the outcome:

• Deprecated the custom Guard provided by this package, deprecated in favor of the new system.
• Both mechanism will be available in until the Guards are removed from Symfony (removed in Symfony 6)