[5.7] Do not pass the guard instance to the authentication events

[rodrigopedra]

PR #25261 breaks usage of queued listeners on authentication events as it passes the Guard instance which can contain closure based inner properties thus is not serializable

This PR changes the behavior to pass only the guard name to the events constructors so:

1. The intended feature fro PR [5.7] Add guard to authentication events #25261 is still present
2. Any queued listeners can be serialized

I am working in the tests right now to prevent future issues

[rodrigopedra]

Changed the tests to ensure the guards are passed as string

[deleugpn]

Although it feels like a breaking change, this is more of a feature fix when queuing.
I will try to work on an integration test that makes sure to never allow closure on queueable objects.

[frigginglorious]

Is there a reason why this was left out of the Upgrade guide at https://laravel.com/docs/5.7/upgrade ?

[deleugpn]

If memory serves me right this was introduced at the end of 5.6 and reverted at the first week of 5.7 or so. I guess that's why it didn't make the list because it was a revert of a barely released feature.