chore: Bump Rust to 1.90

This change moves Rust to version 1.90 in `rust-toolchain.toml`, and also checks in
the `Cargo.lock` file, removing it from `.gitignore`. This is per current Rust
project guidance (from just over two years ago):
<https://blog.rust-lang.org/2023/08/29/committing-lockfiles/>

Author: RobJellinghaus