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/inboxes/fetch_imap_email_inboxes_job.rb

@@ -2,7 +2,7 @@ class Inboxes::FetchImapEmailInboxesJob < ApplicationJob
   queue_as :low
 
   def perform
-    Inbox.where(channel_type: 'Channel::Email').all.each do |inbox|
+    Inbox.where(channel_type: 'Channel::Email').all.find_each(batch_size: 100) do |inbox|
       ::Inboxes::FetchImapEmailsJob.perform_later(inbox.channel) if inbox.channel.imap_enabled
     end
   end