chore: use [lints] to address unexpected_cfgs lint (#7124)

tokio-util/Cargo.toml

@@ -68,3 +68,6 @@ rustc-args = ["--cfg", "docsrs", "--cfg", "tokio_unstable"]
 
 [package.metadata.playground]
 features = ["full"]
+
+[lints]
+workspace = true

tokio-util/src/lib.rs

@@ -1,4 +1,3 @@
-#![allow(unknown_lints, unexpected_cfgs)]
 #![allow(clippy::needless_doctest_main)]
 #![warn(
     missing_debug_implementations,

tokio-util/tests/task_join_map.rs

@@ -1,4 +1,3 @@
-#![allow(unknown_lints, unexpected_cfgs)]
 #![warn(rust_2018_idioms)]
 #![cfg(all(feature = "rt", tokio_unstable))]