mention minimum supported Rust version (rustc ≥ 1.89) in the readme.md

[Aditya30ag]

Problem

When building LibAFL from a fresh setup, users may hit the following error:

Can we add the minimum supported rust version in the readme

If users already had installed the rust but the older version we can add this cmd also in the readme to update the version

rustup update stable

[Aditya30ag]

@tokatoka please let me know if i can work on this or this issue is valuable