[7.x] Add AuthenticationException on Gate

[Oipnet]

Curently :
When a user is not authenticated and is required in the policy an http response with 403 is return

Now :
When a user is not authenticated and is required in the policy an http response with 401 is return

[kadevland]

it's good feature and more logical

[taylorotwell]

Breaking change. May break people's tests, etc.

[kadevland]

I understand the possibilite of Breaking change.
May be if policy have config file and in config file set active ( by default is not active)
"this authenticated and is required and null response with 401 "
so no break change by default.
@taylorotwell @GrahamCampbell

[Oipnet]

@taylorotwell do you think this breaking change can be include in future version like Laravel 8.