That's a way to avoid complicated coordinated upgrades across the ecosystem if certain conditions are met. It's called the "semver-trick" [1].

[1]: https://github.com/dtolnay/semver-trick

Cargo has its own interpretation of semver: first non-zero number is the major version, so v0.6 is like v6.0.0 and v0.7 is like v7.0.0. Then each major version is treated as a separate crate, as if it was cortex-m-v6 and cortex-m-v7.