Time to make new style middlewares default

[asvetlov]

@samuelcolvin you was the author of @middleware syntax (thank you so much for it by the way).

Now we are in the middle of a migration to aiohttp 4.0.

We can drop old-styled middlewares entirely.
@middleware decorator should exist until 5.0 but its functionality must be default I believe.

Would you prepare the PR?

[samuelcolvin]

Will do.

[asvetlov]

Fixed by #3710