trivial: Update Cargo.toml template resolver = "2"

When we try to run `cargo xtask build-ebpf` etc. we get the warning
about Workspace default resolver set to "1". Since all the projects will
be 2021 Edition, adding 'resolver = "2"' in the root `Cargo.toml`
template to avoid this warning.

Cargo.toml

@@ -1,2 +1,3 @@
 [workspace]
 members = ["xtask", "{{project-name}}", "{{project-name}}-common"]
+resolver = "2"