fix: use MSRV aware resolver

[ethanpailes]

This configures cargo to use the MSRV aware resolver, which should hopefully prevent future release woes.

[Aetf]

Also need to update here: https://github.com/shell-pool/shpool/blob/master/.github/workflows/publish.yml#L32 to not use the pinned rust version

[ethanpailes]

I think this will make us continue to work with the pinned toolchain, so we don't need to touch the publish pipeline at all. When I ran cargo package locally with this change applied, it worked.

[Aetf]

Interesting. I thought the config value is only respected since 1.84 https://doc.rust-lang.org/cargo/reference/config.html#resolverincompatible-rust-versions

But if it works, then great :)

[ethanpailes]

Yeah, I find it confusing. Maybe it won't work the next time we cut a release, in which case we can revisit.