Remove @nest from Claro's system-admin--admin-list stylesheet

Problem/Motivation

This is part of the CSS modernization initiative.

Steps to reproduce

Remove @nest from the stylesheet present at https://git.drupalcode.org/project/drupal/-/blob/11.x/core/themes/claro/css/components/system-admin--admin-list.pcss.css

Proposed resolution

@nest was removed from the specification completely. Future versions of postcss-nesting plugin will error if you use @nest.
Refer to this link for further details.

Remaining tasks

Provide before and after screenshots.