Fix feature = "cargo-clippy" deprecation

[rex4539]

https://blog.rust-lang.org/2024/02/28/Clippy-deprecating-feature-cargo-clippy.html

[nical]

Thanks. Reading the blog post it looks like the clippy cfg will arrive in rustc 1.78 which would mean the annotations would be ignored until then.

I think that we can instead replace all of the annotations with the simpler #[allow(clippy::just_underscores_and_digits)] form which should work with euclid's msrv (1.56).

[rex4539]

Sounds good.

[nical]

Thanks!