Fix mdbook build error preventing deploys

[lqd]

Deploys aren't working right now. I don't know when the issue started or what caused it, but it must be recent-ish.

Example GHA failure here:

Error: user name `preprocessor` does not start with `@`
Error: -08 15:25:40 [ERROR] (mdbook::utils): Error: The "goals" preprocessor exited unsuccessfully with exit status: 1 status
error: Recipe `build` failed on line 8 with exit code 101

There's a preprocessor.goals.ignore_users value in the users config, so it's seen as a user in the users config, and causes an error. From the preprocessor code, it looks like it should be a separate config section, with its own key of users to ignore. This PR does that. (I don't know what the ignoring users code does or if it works, this PR just fixes mdbook build)

In the logs above, there's also a warning about the version of mdbook Warning: The goals plugin was built against version 0.4.43 of mdbook, but we're being called from version 0.4.40, so I've also bumped the GHA one to 0.4.43.

(From the logs, I've also noticed there's a cargo rpg json 2024h2 --json-path src/api/2024h2.json step in the justfile, but I don't know what it does and if it needs to be updated for 2025h1?)