file_directory_temp() is deprecated and moved to the FileSystem service

1. file_directory_temp() is deprecated and moved to the FileSystem service.
2. system.file:path.temporary (the path.temporary key-value pair in system.file) is now deprecated.

Before:

file_directory_temp();

After:

\Drupal::service('file_system')->getTempDirectory();

The following lines are now in default.settings.php

/**
 * Temporary file path:
 *
 * A local file system path where temporary files will be stored. This directory
 * must be absolute, outside of the Drupal installation directory and not
 * accessible over the web.
 *
 * If this is not set, the default for the operating system will be used.
 *
 * @see \Drupal\Component\FileSystem\FileSystem::getOsTemporaryDirectory()
 */
# $settings['file_temp_path'] = '/tmp';

Existing sites with custom values should add this instead of $config['system.file']['path']['temporary'] = 'foo/bar'; to their settings.php file before running update.php to ensure the old config at system.file:path.temporary is cleaned up.