Generate crates with Rust 2021 edition

[david-perez]

We currently generate crates using Rust 2018 edition:

https://github.com/awslabs/smithy-rs/blob/2931c9e1e68bfc74618e427ecb6d6a86a6a35a5f/codegen/src/main/kotlin/software/amazon/smithy/rust/codegen/smithy/generators/CargoTomlGenerator.kt#L63-L63

In #1268, runtime crates were updated to Rust 2021 edition.