[SecurityBundle] Outdated mention about services generated by in-memory user provider

[chalasr]

Good first issue: https://symfony.com/doc/4.3/security/user_provider.html#injecting-a-user-provider-in-your-services mentions a security.user.provider.in_memory.user service which is not created anymore since symfony/symfony#25837 (2.7).
It should be removed from the list. The Pull Request should be done against 3.4 branch.

[JoppeDC]

I couldn't help but notice that this issue seems to be only present in docs 4.2 and up. Not sure how/why to PR against 3.4?