facets_preprocess_facets_item_list() is broken in Drupal 11.2 and older

As part of #3589162: Support special hierarchy processors as exposed filters, the (entirely unrelated) change was made to replace the deprecated template_preprocess_item_list() call in facets_preprocess_facets_item_list() with \Drupal::service(ThemePreprocess::class)->preprocessItemList().

However, as that service isn’t available for module versions earlier than Drupal 11.3, this leads to a fatal error for all older versions.
This is trivial to fix using the DeprecationHelper. Alternatively, of course, the 3.x version could declare a minimum Core version of 11.3.

Similar to #3594309: 3.0.x-dev only works on Drupal 11, but affecting even some Drupal 11 versions.