Looks like since I've last looked at the repo, the update to
MSRV 1.56 happened, and we don't have any clippy errors on the 2021
edition, so I _think_ we can just up this value now.

Closes tokio-rs#553

This was introduced in [Rust 1.56](https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html#cargo-rust-version)
so we can set this explicitly here. This means that folks that happen to have an older
rust version set don't run into abstruse compiler errors.