This commit addresses `DEPRECATION WARNING: Defining enums with keyword arguments is deprecated and will be removed`
warning in Action Mailbox.

* Steps to reproduce
```ruby
git clone https://github.com/rails/rails
cd rails/actionmailbox
bundle install
bin/test test/unit/router_test.rb
```

* Without this commit
```
$ bin/test test/unit/router_test.rb
... snip ..
DEPRECATION WARNING: Defining enums with keyword arguments is deprecated and will be removed
in Rails 7.3. Positional arguments should be used instead:

enum :status, [:pending, :processing, :delivered, :failed, :bounced]
 (called from <class:InboundEmail> at /home/yahonda/src/github.com/rails/rails/actionmailbox/app/models/action_mailbox/inbound_email.rb:31)
Run options: --seed 65254

...............

Finished in 0.230357s, 65.1163 runs/s, 108.5271 assertions/s.
15 runs, 25 assertions, 0 failures, 0 errors, 0 skips
$
```

Follow up https://github.com/rails/rails/pull/50987
Refer to https://github.com/rails/rails/pull/51037