Update tokio-util requirement from 0.6 to 0.7 (#1)

Updates the requirements on [tokio-util](https://github.com/tokio-rs/tokio) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](tokio-rs/tokio@tokio-util-0.6.0...tokio-util-0.7.12)

---
updated-dependencies:
- dependency-name: tokio-util
  dependency-type: direct:production
...

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minicbor-io/Cargo.toml

@@ -28,4 +28,4 @@ minicbor-io = { path = ".", features = ["async-io"] }
 quickcheck  = "1.0.1"
 rand        = "0.8"
 tokio       = { version = "1.0", features = ["macros", "net", "rt"] }
-tokio-util  = { version = "0.6", features = ["compat"] }
+tokio-util  = { version = "0.7", features = ["compat"] }