Update async-io requirement from 1.12.0 to 2.1.0

Updates the requirements on [async-io](https://github.com/smol-rs/async-io) to permit the latest version.
- [Release notes](https://github.com/smol-rs/async-io/releases)
- [Changelog](https://github.com/smol-rs/async-io/blob/master/CHANGELOG.md)
- [Commits](smol-rs/async-io@v1.12.0...v2.1.0)

---
updated-dependencies:
- dependency-name: async-io
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -18,7 +18,7 @@ exclude = ["/.*"]
 async-channel = "2.0.0"
 async-executor = "1.5.0"
 async-fs = "1.3.0"
-async-io = "1.12.0"
+async-io = "2.1.0"
 async-lock = "2.6.0"
 async-net = "2.0.0"
 blocking = "1.3.0"