Update cargo_metadata requirement from 0.20 to 0.21

Updates the requirements on [cargo_metadata](https://github.com/oli-obk/cargo_metadata) to permit the latest version.
- [Release notes](https://github.com/oli-obk/cargo_metadata/releases)
- [Changelog](https://github.com/oli-obk/cargo_metadata/blob/main/CHANGELOG.md)
- [Commits](oli-obk/cargo_metadata@0.20.0...0.20.0)

---
updated-dependencies:
- dependency-name: cargo_metadata
  dependency-version: 0.20.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

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"