Update tokio-util requirement from 0.6 to 0.7

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](https://github.com/tokio-rs/tokio/commits)

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

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

tokio-postgres/Cargo.toml

@@ -54,7 +54,7 @@ postgres-protocol = { version = "0.6.1", path = "../postgres-protocol" }
 postgres-types = { version = "0.2.2", path = "../postgres-types" }
 socket2 = "0.4"
 tokio = { version = "1.0", features = ["io-util"] }
-tokio-util = { version = "0.6", features = ["codec"] }
+tokio-util = { version = "0.7", features = ["codec"] }
 
 [dev-dependencies]
 tokio = { version = "1.0", features = ["full"] }