Upgrade workspace to Cargo resolver 2

[iamjpotts]

The crates imply resolver v2 based on their use of edition 2021, but the workspace defaults to v1. Make them match on v2.

Background:

https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html

https://nickb.dev/blog/cargo-workspace-and-the-feature-unification-pitfall/

Also resolves this warning:

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`

[ferristseng]

Thanks!