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/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