suggestion: set rust's edition for exercises to 2021

[Jagholin]

The rustlings crate itself has its edition set in Cargo.toml:

[package] name = "rustlings" version = "4.7.1" authors = ["mokou [email protected]", "Carol (Nichols || Goulding) [email protected]"] edition = "2021"

But for some reason, exercises use a different setting:

https://github.com/rust-lang/rustlings/blob/b19f74e8cfaba7775e8ff2d74cab9cb862fb1473/src/exercise.rs#L124

which is strange, but also confuses rust-analyzer sometimes, because some typings have changed between 2018 and 2021