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/controllers/api/v1/accounts/bulk_actions_controller_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe 'Api::V1::Accounts::BulkActionsController', type: :request do
     create(:conversation, account_id: account.id, status: :open, team_id: team_1.id)
     create(:conversation, account_id: account.id, status: :open)
     create(:conversation, account_id: account.id, status: :open)
-    Conversation.all.each do |conversation|
+    Conversation.all.find_each do |conversation|
       create(:inbox_member, inbox: conversation.inbox, user: agent_1)
       create(:inbox_member, inbox: conversation.inbox, user: agent_2)
     end

spec/jobs/bulk_actions_job_spec.rb

@@ -16,7 +16,7 @@ RSpec.describe BulkActionsJob, type: :job do
   let!(:conversation_3) { create(:conversation, account_id: account.id, status: :open) }
 
   before do
-    Conversation.all.each do |conversation|
+    Conversation.all.find_each do |conversation|
       create(:inbox_member, inbox: conversation.inbox, user: agent)
     end
   end

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