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
...

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

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

substrate/client/authority-discovery/Cargo.toml

@@ -20,7 +20,7 @@ prost-build = "0.9"
 async-trait = "0.1"
 codec = { package = "parity-scale-codec", default-features = false, version = "3.0.0" }
 thiserror = "1.0"
-futures = "0.3.19"
+futures = "0.3.21"
 futures-timer = "3.0.1"
 ip_network = "0.4.1"
 libp2p = { version = "0.40.0", default-features = false, features = ["kad"] }