Relax MSRV to 1.60

crossbeam-rs/crossbeam#1056

Author: taiki-e

Cargo.toml

@@ -6,7 +6,7 @@ name = "event-listener"
 version = "4.0.2"
 authors = ["Stjepan Glavina <[email protected]>"]
 edition = "2021"
-rust-version = "1.61"
+rust-version = "1.60"
 description = "Notify async tasks or threads"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/event-listener"