Check, document and validate the Rust version used by the Ferrocene compiler

We need to determine and document which Rust toolchain version is used by the Ferrocene compiler. This information is essential for compatibility, troubleshooting and ensuring consistent development environments.

Tasks:
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- Check which Rust version is used or bundled with Ferrocene (e.g., via rustc --version or Ferrocene documentation).

- Document the Rust version in a dedicated file/section.

- Note any important differences from upstream Rust (if any).