feat: relax connection IO trait bounds

[ehaydenr]

I can't tell if this was intentional. hyper 0.14.x didn't have this requirement and as far as I can tell it's not actually needed.

[dswij]

For context, I believe that this is here for

hyper/src/server/conn/http1.rs

Line 180 in 4d0c184

pub fn without_shutdown(self) -> impl Future<Output = crate::Result<Parts<I, S>>>

.

But the Connection impl itself, and Dispatch does not require this bound, so it's safe to remove this from impl Future.

[dswij]

Thank you! @ehaydenr