Fix new Rust warning about wrong resolver version

> warning: virtual workspace defaulting to `resolver = "1"` despite one
> or more workspace members being on edition 2021 which implies
> `resolver = "2"`
> 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
> note: for more details see https://doc.rust-lang.org/cargo/reference/
> resolver.html#resolver-versions

So just set resolver = "2" in the root Cargo.toml, as instructed.

Cargo.toml

@@ -3,3 +3,5 @@
 members = [
     "redwood",
 ]
+
+resolver = "2"