Update tokio-tungstenite requirement from 0.20 to 0.21

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
...

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

Cargo.toml

@@ -40,7 +40,7 @@ bytes = "1"
 hyper = {version="1.0", features=["full"], optional=true}
 headers = "0.4"
 hyper-tls = {version="0.6", optional=true}
-tokio-tungstenite = {version="0.20", optional=true}
+tokio-tungstenite = {version="0.21", optional=true}
 
 [dev-dependencies]