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

spec/jobs/inboxes/fetch_imap_email_inboxes_job_spec.rb

@@ -15,10 +15,6 @@ RSpec.describe Inboxes::FetchImapEmailInboxesJob, type: :job do
 
   context 'when called' do
     it 'fetch all the email channels' do
-      imap_email_inboxes = double
-      allow(imap_email_inboxes).to receive(:all).and_return([email_inbox])
-      allow(Inbox).to receive(:where).and_return(imap_email_inboxes)
-
       expect(Inboxes::FetchImapEmailsJob).to receive(:perform_later).with(imap_email_channel).once
 
       described_class.perform_now