Upgrade Rust version to 2021 for Chinese version

chinese https://github.com/sunface/rust-by-practice/blob/e0e88e963a3d2c96029e1e3dca3397d38db78cab/zh-CN/book.toml#L22

English https://github.com/sunface/rust-by-practice/blob/e0e88e963a3d2c96029e1e3dca3397d38db78cab/en/book.toml#L22

Rust play ground version is already 2021 on English pages. Chinese version should have the same version.

And, some solutions don't work under 2018, e.g. this.