Use cargo resolver 2 in workspace config, and remove feature line

This addresses the following warning:

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest

The resolver version 2 was stabilised ages ago as part of Rust 2021, and we're already using it in some crates as the message says, so this shouldn't cause any issues and gets rid of the warning.

(Also, got rid of a cargo-features line that I'm pretty sure was only relevant pre Rust 2021 and only worked on nightly)