Update tokio-tungstenite requirement from 0.20.1 to 0.21.0

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@v0.20.1...v0.21.0)

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

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

pingora-proxy/Cargo.toml

@@ -43,7 +43,7 @@ tokio-test = "0.4"
 env_logger = "0.9"
 hyperlocal = "0.8"
 hyper = "0.14"
-tokio-tungstenite = "0.20.1"
+tokio-tungstenite = "0.21.0"
 pingora-load-balancing = { version = "0.1.0", path = "../pingora-load-balancing" }
 prometheus = "0"
 futures-util = "0.3"