textproc/mdbook: Update to 0.4.21
Changes:
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0416 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0417 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0418 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0419 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0420 https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0421
PR: 267007
Approved by: 0mp (maintainer timeout)