Updates should really test the reverse dependencies, especially if
there's only one.

> Perhaps a specific message should be added for migration to the version 0.5 (see breaking changes in https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#05-migration-guide).

The changelog has:

* Removed the book.multilingual field. This was never used.

Except that in-tree rust-analyzer uses it. I think the easiest fix will
be to update to the latest rust-analyzer.

Finished `release` profile [optimized] target(s) in 32m 25s
cd /work/pobj/rust-analyzer-20250512/rust-analyzer-2025-05-12/docs/book && /usr/bin/env -i CFG_RELEASE=2025-05-12 PORTSDIR="/usr/ports" LIBTOOL="/usr/bin/libtool"  PATH='/work/pobj/rust-analyzer-20250512/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11R6/bin' PREFIX='/usr/local'  LOCALBASE='/usr/local' X11BASE='/usr/X11R6'  CFLAGS='-O2 -pipe'  TRUEPREFIX='/usr/local' DESTDIR=''  HOME='/rust-analyzer-20250512_writes_to_HOME' PICFLAG="-fpic"  BINGRP=bin BINOWN=root BINMODE=755 NONBINMODE=644  DIRMODE=755  INSTALL_COPY=-c INSTALL_STRIP=-s  MANGRP=bin MANOWN=root MANMODE=644 BSD_INSTALL_PROGRAM="/work/pobj/rust-analyzer-20250512/bin/install -c -s -m 755"  BSD_INSTALL_SCRIPT="/work/pobj/rust-analyzer-20250512/bin/install -c -m 755"  BSD_INSTALL_DATA="/work/pobj/rust-analyzer-20250512/bin/install -c -m 644"  BSD_INSTALL_MAN="/work/pobj/rust-analyzer-20250512/bin/install -c -m 644"  BSD_INSTALL_PROGRAM_DIR="/work/pobj/rust-analyzer-20250512/bin/install -d -m 755"  BSD_INSTALL_SCRIPT_DIR="/work/pobj/rust-analyzer-20250512/bin/install -d -m 755"  BSD_INSTALL_DATA_DIR="/work/pobj/rust-analyzer-20250512/bin/install -d -m 755"  BSD_INSTALL_MAN_DIR="/work/pobj/rust-analyzer-20250512/bin/install -d -m 755"  /usr/local/bin/mdbook build
ERROR Invalid configuration file
	Caused by: TOML parse error at line 4, column 1
  |
4 | multilingual = false
  | ^^^^^^^^^^^^
unknown field `multilingual`, expected one of `title`, `authors`, `description`, `src`, `language`, `text-direction`