[Security] rename User to InMemoryUser

[xabbuh]

fixes #15111

[javiereguiluz]

Thanks for this contribution!

Given that this is an important change, and to make the transition smoother to users (and to Google search results) we may add a comment about how this class was renamed. We do the same in other parts of the docs, e.g.:

.. versionadded:: 4.3

    The :class:`Symfony\\Component\\HttpKernel\\Event\\ExceptionEvent` class was
    introduced in Symfony 4.3. In previous versions it was called
    ``Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent``.

[wouterj]

I would do this as a code comment: // The InMemoryUser class was introduced in Symfony 5.3. In previous versions it was called User. (IIRC, we also applied that tactic for some renames that are often used in non-related code)

[javiereguiluz]

Thanks Christian! I've added the comments as suggested by Wouter and they look nice. Cheers!

[chalasr]

Big thanks to Christian and reviewers.