Update tokio-tungstenite requirement from 0.20 to 0.21 (#639)

Updates the requirements on [tokio-tungstenite](https://github.com/snapview/tokio-tungstenite) to permit the latest version.
- [Changelog](https://github.com/snapview/tokio-tungstenite/blob/master/CHANGELOG.md)
- [Commits](snapview/tokio-tungstenite@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: tokio-tungstenite
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/websocket/Cargo.toml

@@ -9,7 +9,7 @@ edition = "2018"
 [dependencies]
 gotham = { path = "../../gotham" }
 futures-util = "0.3.14"
-tokio-tungstenite = "0.20"
+tokio-tungstenite = "0.21"
 tokio = "1.11.0"
 pretty_env_logger = "0.5"
 sha1 = "0.10"