Cargo.lock version=4 breaks backward compatibility with Rust < 1.78

- hf_transfer 0.1.9
- Python 3.11 / 3.12
- Rust 1.75

The commit https://github.com/huggingface/hf_transfer/commit/16bbc5045ff4036be9d8bcd0050108d82229a216 bumped the `Cargo.lock` version from v3 to v4. The change is not present in the main branch, only in the commit that leads to [v0.1.9](https://github.com/huggingface/hf_transfer/tree/v0.1.9) tag. The bump to `version = 4` broke backwards compatibility with Rust < 1.78. `Cargo.lock` v4 was introduced in Rust 1.78 and became default in 1.83.

From Cargo [changelog](https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md):
> Cargo 1.83 (2024-11-28):
> Lockfile format v4 is now the default for creating/updating a lockfile. Rust toolchains 1.78+ support lockfile v4. For compatibility with earlier MSRV, consdier setting the package.rust-version to 1.82 or earlier.

`hf_transfer` builds fine with older Rust version. I'm rebuilding `hf_transfer` from source on a platform with Rust 1.75. I guess you created the 0.1.9 release on a machine with Rust 1.83.

Would you accept a PR that adds MSRV to `Cargo.toml` and updates `CI.yaml` to test with that MSRV? MSRV can be at least 1.75, maybe older.