Bump futures-util from 0.3.19 to 0.3.21 (#4883)

Bumps [futures-util](https://github.com/rust-lang/futures-rs) from 0.3.19 to 0.3.21.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.19...0.3.21)

---
updated-dependencies:
- dependency-name: futures-util
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

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

node/zombienet-backchannel/Cargo.toml

@@ -12,7 +12,7 @@ publish = false
 tokio = { version = "1.0.0", default-features = false, features = ["macros", "net", "rt-multi-thread", "sync"] }
 url = "2.0.0"
 tokio-tungstenite = "0.16"
-futures-util = "0.3.18"
+futures-util = "0.3.21"
 lazy_static = "1.4.0"
 parity-scale-codec = { version = "2.3.1", features = ["derive"] }
 reqwest = "0.11"