Update minimum mdbook version to 0.4.21.

[tjni]

This will fix compilation using Rust 1.64.

If you have time, please take a look. Incorporating this and publishing a new release (with Cargo.lock) will simplify how the application is packaged in nixpkgs, which is updating Rust in its ecosystem to 1.64. In the meantime, I'll work around it there.

Thank you for your work.