crate doesn't have to depend on 2018/2021 editions of Rust
This fixes builds for crates that have to support legacy legacy systems:
error: failed to parse manifest at `.../.cargo/registry/src/github.com-1ecc6299db9ec823/unwrap_or-1.0.0/Cargo.toml`
Caused by:
failed to parse the `edition` key
Caused by:
this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.