[Security] Add deprecation information for is_anonymous

[malteschlueter]

Don't know in which issue or pr this happend but I found in the UPGRADE-5.4.md that the is_anonymous() function is deprecated. Instead the is_authenticated() function should be used.

https://github.com/symfony/symfony/blob/5.4/UPGRADE-5.4.md#security

Deprecate AuthenticationTrustResolverInterface::isAnonymous() and the is_anonymous() expression function as anonymous no longer exists in version 6, use the isFullFledged() or the new isAuthenticated() instead if you want to check if the request is (fully) authenticated

How do we handle the upmerge? The deprecation information must be removed in 6.0 and the general information about the function in line 67-70. Should I create a follow up PR?

[javiereguiluz]

Thanks Malte ... and sorry it took us so long to merge this. Yes, when we merge this into upper branches, we'll remove the deprecated feature, so there's no need for other PRs. Thanks!