Refactor template_preprocess_forum_list() to not manipulate NodeInterface objects

Forum is approved for removal. See #1898812: [policy] Deprecate forum module for removal in Drupal 11 →

This is now Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project → and the Extensions approved for removal → policies.

It will be moved to the contributed extension → once the Drupal 11 branch is open.