Update all crates to Rust edition 2021

[romac]

Description

The 2021 edition of the Rust language has been out for a couple years now and enables a bunch of new features and idioms that make the code clearer and often shorter (see the link above for more details).

As part of fixing the new clippy warnings emitted in Rust 1.67, which was released today, I noticed a bunch of our crates are still on edition 2018, which makes it impossible to fix the warnings without outright ignoring them. The tendermint crate and a couple others are already on edition 2021.

I would therefore suggest we update all crates to use edition 2021.

Definition of "done"

All crates in the repository use the 2021 edition of the Rust language.