Use rust-version in Cargo.toml

[letFunny]

Is there any reason why we are not setting the rust-version in Cargo.toml? I am using a crate (cargo-llvm-cov) that depends on ruzstd and my CI broke recently because 0.8.2 started using is_multiple_of which is only stable in Rust 1.87, while my CI was using the 1.85 version.