Update tokio-util requirement from 0.4 to 0.5

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/tokio-util-0.5.0)

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

Author: dependabot-preview[bot]

tokio-postgres/Cargo.toml

@@ -51,7 +51,7 @@ postgres-protocol = { version = "0.5.0", path = "../postgres-protocol" }
 postgres-types = { version = "0.1.2", path = "../postgres-types" }
 socket2 = "0.3"
 tokio = { version = "0.3", features = ["io-util"] }
-tokio-util = { version = "0.4", features = ["codec"] }
+tokio-util = { version = "0.5", features = ["codec"] }
 
 [dev-dependencies]
 tokio = { version = "0.3", features = ["full"] }