Use mdbook v0.4.21

[drbrain]

On rust 1.64.0 mdbook v0.4.18 fails to compile:

   Compiling mdbook v0.4.18
error[E0597]: `local_ctx` does not live long enough
   --> /Users/erichodel/.cargo/registry/src/github.com-1ecc6299db9ec823/mdbook-0.4.18/src/renderer/html_handlebars/helpers/navigation.rs:154:25
    |
154 |             t.render(r, &local_ctx, &mut local_rc, out)
    |                         ^^^^^^^^^^ borrowed value does not live long enough
155 |         })?;
    |         -
    |         |
    |         `local_ctx` dropped here while still borrowed
    |         borrow might be used here, when `local_rc` is dropped and runs the destructor for type `handlebars::RenderContext<'_, '_>`
    |
    = note: values in a scope are dropped in the opposite order they are defined

For more information about this error, try `rustc --explain E0597`.
error: could not compile `mdbook` due to previous error
warning: build failed, waiting for other jobs to finish...

[wez]

Can you publish a release with this change?

[badboy]

Can you publish a release with this change?

I'm on vacation for another week, I will get to a release when I'm back.