Update workspace resolver version to 3 (#915)

For virtual workspaces (workspaces where there isn't a top level crate,
only member crates), there is no top-level Rust `edition` property from
which Cargo can infer its resolver version. Instead, one must specify the
resolver version explicitly.

Now we're using Rust 2024 edition (which defaults to resolver version
3 for crates using that edition), we should update the overall workspace
resolver version from 2 to 3 for parity with it.

See:
https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

GUS-W-17894359.

Author: edmorley

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-resolver = "2"
+resolver = "3"
 members = [
     "examples/basics",
     "examples/execd",