Specify MSRV by using `rust-version` in `Cargo.toml`

[hellow554]

Currently, the readme states:

Bincode 2.0 is still in development and does not yet have a targeted MSRV. Once 2.0 is fully released the MSRV will be locked. After this point any changes to the MSRV are considered a breaking change for semver purposes.

I would suggest to specify the MSRV in Cargo.toml by using rust-version field as soon as a MSRV is selected so tools like cargo and clippy work better with the code.