Incorrect note in authentication recipe doc

Hello!

From [authentication recipe](https://gqlgen.com/recipes/authentication/):

> Subscriptions are long lived, if your tokens can timeout or need to be refreshed you should keep the token in context too and verify it is still valid in `auth.ForContext`.

This seems incorrect. If I'm not wrong, once the subscription has been set then there is no way to call `auth.ForContext` from the subscriber's context. And as rightly pointed out by that note - subscriptions are long lived. So the token could expire meanwhile and subscription would have no way to know that.

Fortunately, @RobinCPel has pushed the code to close websocket when context is done/cancelled. So we can use `context.WithDeadline` as [suggested](https://github.com/99designs/gqlgen/issues/774#issuecomment-980033736) by them:
```go
func WebsocketAuthInitFunc(ctx context.Context, initPayload transport.InitPayload) (context.Context, error) {
...
newCtx, _ := context.WithDeadline(transport.AppendCloseReason(ctx, "authentication token has expired"), time.Unix(token.ExpiresAt, 0))
return newCtx, nil
}
```

Let's add that to the authentication recipe code and remove above note. (I'm also interested in knowing how to discard the cancel function and silence the vet)