Remove last usage of setAccessible()

Problem/Motivation

The Reflection*::setAccessible() is deprecated and throws deprecation warning since PHP 8.5 beta1.
General usage was removed in 📌 Remove usage of setAccessible() when core requires PHP 8.1 Fixed and 📌 Remove remaining usage of setAccessible() Active

Steps to reproduce

drupal$ git grep setAccessible
core/tests/Drupal/Tests/Core/Theme/Icon/IconPackManagerTest.php:91:    $method->setAccessible(TRUE);

Proposed resolution

- remove remaining usage

Remaining tasks

MR, review, commit

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet