Update to edition 2021

[Juan Hernández]

Update Cargo.toml with:

[package]
edition = "2021"
rust-version = "1.56"

• https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html
• https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html