Fix build error with -Z minimal-versions

```
error[E0635]: unknown feature `proc_macro_span_shrink`
--> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.52/src/lib.rs:92:30
|
92 | feature(proc_macro_span, proc_macro_span_shrink)
| ^^^^^^^^^^^^^^^^^^^^^^
```

async-stream-impl/Cargo.toml

@@ -12,7 +12,7 @@ repository = "https://github.com/tokio-rs/async-stream"
 proc-macro = true
 
 [dependencies]
-proc-macro2 = "1"
+proc-macro2 = "1.0.60"
 syn = { version = "2.0.2", features = ["full", "visit-mut"] }
 quote = "1"