Bump MSRV to 1.60

crossbeam-rs/crossbeam#1056

```
error: package `crossbeam-utils v0.8.18` cannot be built because it requires rustc 1.60 or newer, while the currently active rustc version is 1.59.0
```

Cargo.toml

@@ -4,7 +4,7 @@ name = "event-listener-strategy"
 version = "0.4.0"
 edition = "2018"
 authors = ["John Nunley <[email protected]>"]
-rust-version = "1.59"
+rust-version = "1.60"
 description = "Block or poll on event_listener easily"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/event-listener"