Update cargo_metadata requirement from 0.20 to 0.21 (#411)

Author: loongs-zhang

Cargo.toml

@@ -27,7 +27,7 @@ open-coroutine-macros = { path = "macros", version = "0.7.0" }
 tracing = { version = "0.1", default-features = false }
 tracing-subscriber = { version = "0.3", default-features = false }
 tracing-appender = { version = "0.2", default-features = false }
-cargo_metadata = { version = "0.20", default-features = false }
+cargo_metadata = { version = "0.21", default-features = false }
 mio = { version = "1.0", default-features = false }
 
 cfg-if = "1.0.0"