PERF: Update UserDestroyer to fetch histories and actions in batch (#16316)

This is much more memory-efficient when there are 1000s of rows

app/services/user_destroyer.rb

@@ -40,14 +40,14 @@ class UserDestroyer
         delete_posts(user, category_topic_ids, opts)
       end
 
-      user.post_actions.each do |post_action|
+      user.post_actions.find_each do |post_action|
         post_action.remove_act!(Discourse.system_user)
       end
 
       # Add info about the user to staff action logs
       UserHistory.staff_action_records(
         Discourse.system_user, acting_user: user.username
-      ).each do |log|
+      ).find_each do |log|
         log.details ||= ''
         log.details = (log.details.split("\n") +
             ["user_id: #{user.id}", "username: #{user.username}"]