Update mdbook requirement from 0.3.1 to 0.4.12

[dependabot-preview[bot]]

Updates the requirements on mdbook to permit the latest version.

Release notes

Sourced from mdbook's releases.

v0.4.12

This fixes a regression in 0.4.11 which broke hidden code line removal.

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0412 for a complete list of changes.

Changelog

Sourced from mdbook's changelog.

mdBook 0.4.12

14add9c...8b4e488

Changed

• Reverted the change to update to highlight.js 11, as it broke hidden code lines. #1597

mdBook 0.4.11

e440094...2cf00d0

Added

• Added support for Rust 2021 edition. #1596
• Added mdbook completions subcommand which provides shell completions. #1425
• Added --title and --ignore flags to mdbook init to avoid the interactive input. #1559

Changed

• If running a Rust example does not have any output, it now displays the text "No output" instead of not showing anything. #1599
• Code block language tags can now be separated by space or tab (along with commas) to match the behavior of other sites like GitHub and rustdoc. #1469
• Updated warp (the web server) to the latest version. This also updates the minimum supported Rust version to 1.46. #1612
• Updated to highlight.js 11. This has various highlighting improvements. #1597

Fixed

• Inline code blocks inside a header are no longer highlighted when output.html.playground.editable is true. #1613

mdBook 0.4.10

2f7293a...dc2062a

Changed

• Reverted breaking change in 0.4.9 that removed the __non_exhaustive marker on the Book struct. #1572
• Updated handlebars to 4.0. #1550
• Removed the chapter_begin id on the print page's chapter separators. #1541

mdBook 0.4.9

... (truncated)

Commits

• e6629cd Merge pull request #1623 from ehuss/release-next
• 5a077b9 Update to 0.4.12
• 8b4e488 Merge pull request #1621 from ehuss/revert-highlightjs11
• 68d8cee Revert #1597 - Update to highlight.js 11.0.
• db337d4 Merge pull request #1616 from joshrotenberg/add_cleanup_contributors
• 5e27714 add a couple, clean up a little
• 14add9c Merge pull request #1615 from ehuss/release-next
• 87877a9 Update to 0.4.11
• 2cf00d0 Merge pull request #1597 from ehuss/update-hightlight.js
• 8c7af3c Merge pull request #1614 from joshrotenberg/code_of_conduct
• Additional commits viewable in compare view

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