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/migration/update_first_response_time_in_reporting_events_job.rb

@@ -6,7 +6,7 @@ class Migration::UpdateFirstResponseTimeInReportingEventsJob < ApplicationJob
 
   def perform(account)
     get_conversations_with_bot_handoffs(account)
-    account.reporting_events.where(name: 'first_response').each do |event|
+    account.reporting_events.where(name: 'first_response').find_each do |event|
       conversation = event.conversation
 
       # if the conversation has a bot handoff event, we don't need to update the response_time