cli: doctor command checks Rust version against MIN_RUST_MAJOR=1 MIN_RUST_MINOR=56 which is outdated

[Haroldwonder]

Problem

The doctor command validates the Rust toolchain version against MIN_RUST_MAJOR=1, MIN_RUST_MINOR=56. Soroban SDK 21.7.0 requires Rust 1.74+. The minimum version check will pass for Rust 1.56-1.73, which cannot compile the project.

Location

src/bin/anchorkit.rs — MIN_RUST_MAJOR, MIN_RUST_MINOR

Fix

Update MIN_RUST_MINOR to 74 (or the actual minimum required by soroban-sdk 21.7.0) and add a test that the doctor command fails for older versions.