bindings: rust: fix unclear resolver warning

bindings: rust: fix unclear resolver warning

Fix the following warning:

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

Clarify the resolver selection as the 2021 edition by setting the
workspace.resolver to "2".

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Erik Schilling <erik.schilling@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>