Deprecated theme suggestions cause fatal error.

Problem/Motivation

Deprecated theme suggestions can be added through the `__DEPRECATED`-key when adding suggestions.
This causes a fatal error in conjunction with the Preprocess-module, in the `PreprocessManager::preprocess` function we loop over the $suggestions and check if each theme_registry has the given suggestion, but the `__DEPRECATED` key is also available in the `$suggestions`-array, and the `__DEPRECATED` key is an array that contains one or more deprecated theme suggestions.
The cause of this issue, for us, was: https://www.drupal.org/node/3541462

Steps to reproduce

- Install preprocess on a Drupal 11.3 site
- See the error appear whenever a node inside a view gets preprocessed

Proposed resolution

Unset the `__DEPRECATED` key like the ThemeManager does during rendering (\Drupal\Core\Theme\ThemeManager::render() on line 224 in Drupal 11.3.10) like so:

    if (isset($suggestions['__DEPRECATED'])) {
      unset($suggestions['__DEPRECATED']);
    }