chore: Use find_each instead of .all.each (#6975)

- Enable the rubocop Rails/FindEach
- Replace the .all.each with .find_each

This should let us avoid potential memory usage.
Motivation from the speedshop newsletter by Nate Berkopec

ref: https://www.rubyinrails.com/2017/11/16/use-find-each-instead-of-all-each-in-rails/
ref: https://linear.app/chatwoot/issue/CW-1480/chore-run-all-sidekiq-jobs-async

app/jobs/trigger_scheduled_items_job.rb

@@ -3,7 +3,8 @@ class TriggerScheduledItemsJob < ApplicationJob
 
   def perform
     # trigger the scheduled campaign jobs
-    Campaign.where(campaign_type: :one_off, campaign_status: :active).where(scheduled_at: 3.days.ago..Time.current).all.each do |campaign|
+    Campaign.where(campaign_type: :one_off,
+                   campaign_status: :active).where(scheduled_at: 3.days.ago..Time.current).all.find_each(batch_size: 100) do |campaign|
       Campaigns::TriggerOneoffCampaignJob.perform_later(campaign)
     end