Update tungstenite requirement from 0.20.1 to 0.21.0 in /gremlin-client

Updates the requirements on [tungstenite](https://github.com/snapview/tungstenite-rs) to permit the latest version.
- [Changelog](https://github.com/snapview/tungstenite-rs/blob/master/CHANGELOG.md)
- [Commits](snapview/tungstenite-rs@v0.20.1...v0.21.0)

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

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

gremlin-client/Cargo.toml

@@ -43,7 +43,7 @@ chrono = { version = "0.4", default-features = false}
 lazy_static = "1.3.0"
 base64 = "0.21.4"
 native-tls = "0.2.3"
-tungstenite = { version = "0.20.1", features = ["native-tls"] }
+tungstenite = { version = "0.21.0", features = ["native-tls"] }
 async-tungstenite = { version = "0.23", optional = true, default-features=false}
 async-std =  { version = "1.4.0", optional = true, features = ["unstable","attributes"] }
 async-trait = { version = "0.1.10", optional = true }