[6.x] Update logged_in to use the guard from the Statamic config files

[vmitchell85]

This PR updates the logged_in variable to use the correct guard from the users.php config file.

I didn't add any test as I couldn't determine a good way to do that (it's also a bit late so the brain might figure it out in the morning).

I don't expect this to break anything. I ran into this while using logged_in after following the guide regarding Using an Independent Authentication Guard.

[jasonvarga]

Since this does change some behavior potentially, I've changed this to target v6.

In the mean time, you can add a variable yourself. Throw this into your app service provider:

\Illuminate\Support\Facades\View::composer('*', function ($view) {
    $view->with(['logged_into_guard' => auth('yourguard')->check()]);
});

And use logged_into_guard instead of logged_in.