Decide on an MSRV

[knutwalker]

around 1.59 makes sense, set it in Cargo.toml. Update CI to use a Cargo.msrv.lock. Update dependencies accordingly

[knutwalker]

We will go with 1.60 for now, it's the lowest one that is satisfied by all current dependencies. It's gonna 8 releases old tomorrow, which is almost one year, that seems reasonable enough. We also follow this rust-lang guideline with respect to what semver change is required for an MSRV change (it's a minor one).