Implement a Minimum Supported Version of Rust (MSRV) policy

[Enet4]

While this still isn't high on my priorities, the various library crates in the project should eventually have an MSRV policy to prevent surprises caused by the use of old versions of the compiler.

How and when this policy is yet to be decided. Until then, discussion is welcome here.

[Stargateur]

with Rustc 1.56.0 and so Rust edition 2021 allow:

edition = "2021"
rust-version = "1.56"

I think it's a good start.