Rename user system => authentication/authorization system

Problem/Motivation

I see #3385864: [Plan] Simplify core component list, but that seems like a larger scope. As the newly minted maintainer for auth/authZ, I can't manage anything. I'd like to be able to look for issues related to this area of drupal core.

Steps to reproduce

Proposed resolution

I proposed in https://drupal.slack.com/archives/C07V7UZNVSM/p1770645570382859 that we rename the existing user system to auth/authz.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet