Backport #7721 to v26

[torokati44]

Like #8374, but for the 26 series.

Very similar merge conflict situation - tired to make those diffs look as similar to the originals as possible when resolving.

[ErichDonGubler]

This needs to cherry-pick a few things:

1. 83a0722 and its supporting infra changes, so the fails-if directives added by tests here can parse correctly.
2. For the cargo-deny checks, there's other stuff to backport. I think we can just ignore these, since downstream crates already need to handle this in v26.
3. For the MSRV, there may be something to backport, but I'm not sure. I'm not familiar with this arbitrary's interaction with MSRV over its releases. Maybe @bushrat011899 or @cwfitzgerald knows?

I've pushed 9f40cd9 to the backport-pr7721-to-v26 in my erichdongubler-mozilla fork. Feel free to adopt it and force-push here. This takes care of (1) and (2), but I don't have bandwidth to figure out (3). Good luck! 🫡

[torokati44]

@ErichDonGubler

Feel free to adopt it and force-push here. This takes care of (1) and (2),

Thank you, done!

[torokati44]

I also had to backport #8112 (cherry-pick 5762102) to fix the MSRV CI failure with half and arbitrary.
See also: https://github.com/VoidStarKat/half-rs/blob/main/CHANGELOG.md#260---2025-04-08-

[ErichDonGubler]

Nice work!