Bump futures from 0.3.19 to 0.3.21 (#11133)

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
...

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

substrate/client/cli/Cargo.toml

@@ -16,7 +16,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 chrono = "0.4.10"
 clap = { version = "3.1.6", features = ["derive"] }
 fdlimit = "0.2.1"
-futures = "0.3.19"
+futures = "0.3.21"
 hex = "0.4.2"
 libp2p = "0.40.0"
 log = "0.4.11"