Update async-io requirement from 1.6 to 2.0

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@v1.6.0...v2.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

quinn/Cargo.toml

@@ -33,7 +33,7 @@ codecov = { repository = "djc/quinn" }
 maintenance = { status = "experimental" }
 
 [dependencies]
-async-io = { version = "1.6", optional = true }
+async-io = { version = "2.0", optional = true }
 async-std = { version = "1.11", optional = true }
 bytes = "1"
 # Enables futures::io::{AsyncRead, AsyncWrite} support for streams