Enable to use the edition 2021 resolver whole the workspace

[tetsuharuohzeki]

This fixes the warning shown on running cargo build at first.

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

lib/Cargo.toml uses rust edition 2021 for a long time.

So I think resolver = "2" is nice to suppress this warning.