Bump MSRV to 1.60

crossbeam-rs/crossbeam#1056

Author: taiki-e

Cargo.toml

@@ -6,7 +6,7 @@ name = "async-channel"
 version = "2.1.1"
 authors = ["Stjepan Glavina <stjepang@gmail.com>"]
 edition = "2018"
-rust-version = "1.59"
+rust-version = "1.60"
 description = "Async multi-producer multi-consumer channel"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/async-channel"