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