build(deps): Update async-std-resolver requirement from 0.20 to 0.21

Updates the requirements on [async-std-resolver](https://github.com/bluejekyll/trust-dns) to permit the latest version.
- [Release notes](https://github.com/bluejekyll/trust-dns/releases)
- [Changelog](https://github.com/bluejekyll/trust-dns/blob/main/CHANGELOG.md)
- [Commits](bluejekyll/trust-dns@v0.20.0...v0.21.1)

---
updated-dependencies:
- dependency-name: async-std-resolver
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

transports/dns/Cargo.toml

@@ -15,7 +15,7 @@ libp2p-core = { version = "0.32.0", path = "../../core", default-features = fals
 log = "0.4.1"
 futures = "0.3.1"
 trust-dns-resolver = { version = "0.20", default-features = false, features = ["system-config"] }
-async-std-resolver = { version = "0.20", optional = true }
+async-std-resolver = { version = "0.21", optional = true }
 smallvec = "1.6.1"
 
 [dev-dependencies]