Bump futures from 0.3.19 to 0.3.21 (#4886)

Bumps [futures](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](https://github.com/rust-lang/futures-rs/compare/0.3.19...0.3.21)

---
updated-dependencies:
- dependency-name: futures
  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>

polkadot/node/subsystem-util/Cargo.toml

@@ -7,7 +7,7 @@ description = "Subsystem traits and message definitions"
 
 [dependencies]
 async-trait = "0.1.52"
-futures = "0.3.19"
+futures = "0.3.21"
 itertools = "0.10"
 parity-scale-codec = { version = "2.3.1", default-features = false, features = ["derive"] }
 pin-project = "1.0.9"
@@ -33,7 +33,7 @@ sp-keystore = { git = "https://github.com/paritytech/substrate", branch = "maste
 [dev-dependencies]
 assert_matches = "1.4.0"
 env_logger = "0.9.0"
-futures = { version = "0.3.19", features = ["thread-pool"] }
+futures = { version = "0.3.21", features = ["thread-pool"] }
 log = "0.4.13"
 polkadot-node-subsystem-test-helpers = { path = "../subsystem-test-helpers" }
 lazy_static = "1.4.0"