Increase supported Rust version to 1.88.0 (from 1.87.0)

Add support for Rust version 1.88.0 (https://blog.rust-lang.org/2025/06/26/Rust-1.88.0/)

Rust 1.88.0 introduced support for let-chains which we use heavily in our code base.

With the current supported versions, we cannot use codex for our project.