Update actions/checkout to v6.0.3

// This program demonstrates the issue that the v6.0.3 update fixes.
// In a real CI workflow, the actions/checkout action would initialize
// a repository that uses the SHA‑256 hash algorithm.  The old
// implementation would fail to set up the repository correctly.
// Here we simply print a message to illustrate the problem that
// the update resolves.

fn main() {
    println!("If you run actions/checkout v6.0.2 on a SHA‑256 repo, it may fail to init.");
    println!("Upgrading to v6.0.3 fixes that initialization bug and expands merge‑commit SHA matching.");
}