[6.x] Update upgrade guide to include HandlesAuthorization trait change

[ankurk91]

Hi,

This PR adds instructions for a change in Illuminate\Auth\Access\HandlesAuthorization trait that was introduced in this pull request.
laravel/framework#29117

Laravel 5.8 behaviour

The deny() method throws an exception and there is no need of return statement.
The return is useless here knowing that it will throw exception.

    public function update(User $user, Post $post)
    {
        if($user->role !== 'editor') {
            // works without return
            $this->deny("You must be an editor to edit a post.")
        }
        
        return $user->id === $post->user_id;
    }

Laravel 6.0 behaviour

The deny() method no longer throws an exception, developer need to add a return statement in order to make the policy work.
Without the return statement, all of the deny() calls are being ignored.

    public function update(User $user, Post $post)
    {
        if($user->role !== 'editor') {
           // need to add return
            return $this->deny("You must be an editor to edit a post.")
        }
        
        return $user->id === $post->user_id;
    }

Feel free to suggest edits.