Ref. solidusio/solidus#5042

Adapt to new Solidus default branch name

Previously relied on solidus_frontend defining `spree.root_path`, and
used it redirecting admin users to the store home page.

Ensure the admin sign in root path fallback is '/admin'

Version 2.5.8

* tag 'v2.5.8':
  Release v2.5.8

Adapt to the new default branch name of `main`

Update the admin logout feature spec to accommodate the UI changes 
in the admin navigation bar.
The 'Logout' link is now nested within a 'details' tag and must
be clicked/expanded to access the logout functionality.

```
config/initializers/warden.rb:7:99: C: [Correctable] Rails/FindEach: Use find_each instead of each.
```

…avbar-update

Fix failing spec for admin logout

Without this, specs fail with
```
AbstractController::ActionNotFound:
        The new action could not be found for the :authorize_actions
        callback on Spree::UsersController, but it is listed in the controller's
        :only option.
```

I guess this is because the views are not in the standard place, thus
impeding Rails' ability to dynamically generate empty actions if
matching views are present. Anyways: This is good, because it's more
explicit.

[Rails 7.1] Add empty :new action to Spree::UsersController