Switch to Cargo.lock v4

[nspin]

Rust 1.82 makes this the default: rust-lang/cargo#14595

We can't switch until Verus moves to Rust 1.78+ (in progress upstream: verus-lang/verus#1173).

For now, an MSRV for the sel4 crate keeps this project's Cargo.lock at v3.

[nspin]

Addressed by #268