Define MSRV policy

[xtqqczze]

Since the crate was transferred, and beginning with commit db0fc00, the Minimum Supported Rust Version (MSRV) has been increased from 1.65 to 1.87.

At present, the MSRV is not explicitly specified in Cargo.toml. We should clarify and formalize our MSRV policy moving forward.

cc: @sylvestre

[sylvestre]

same as coreutils :)

[xtqqczze]

Maybe we don't want to force downstream crates to bump MSRV, e.g. https://crates.io/crates/crypto-primes?

[xtqqczze]

We do need at least 1.70 due to lru dependency.

[xtqqczze]

same as coreutils :)

I think we could go and publish the next 0.5.0 release, and make backports later if anyone needs them.

[sylvestre]

sure, will do when i have permissions to do it

[xtqqczze]

turned out very little change was needed to reduce MSRV to 1.70, opened #58

[xtqqczze]

I think next release can be 0.4.5 unless there are breaking changes. No need for "major" version for small MSRV bump.