Add MSRV

[valenting]

We recently hit an issue where code in this crate was using a feature only available for rust 1.88+ but that wasn't immediately obvious.
Adding an explicit rust-version to Cargo.toml and enforcing that in CI would be great.

[crowlKats]

THe CI already enforces the version specified in rust-toolchain.toml, but if adding the rust-version helps, we can add it