[Documentation][Voter] updated documentation for CacheableVoterInterface

[dylan-b-efficience-it]

I noticed that the Voter documentation, in particular the code illustrating the point "The Voter Interface", was not update. To explain further, in the Voter.php file on GitHub, we've also been implementing "CacheableVoterInterface" since Symfony 5.4.

So I'd like to suggest this change to the documentation.