chore: upgrade remaining 2018 edition crates to 2021 edition (#5788)

tests-integration/Cargo.toml

@@ -2,7 +2,7 @@
 name = "tests-integration"
 version = "0.1.0"
 authors = ["Tokio Contributors <[email protected]>"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [[bin]]

tests-integration/tests/process_stdio.rs

@@ -7,7 +7,6 @@ use tokio::process::{Child, Command};
 use tokio_test::assert_ok;
 
 use futures::future::{self, FutureExt};
-use std::convert::TryInto;
 use std::env;
 use std::io;
 use std::process::{ExitStatus, Stdio};