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/beefy/rpc/Cargo.toml

@@ -8,7 +8,7 @@ repository = "https://github.com/paritytech/substrate"
 description = "RPC for the BEEFY Client gadget for substrate"
 
 [dependencies]
-futures = "0.3.19"
+futures = "0.3.21"
 log = "0.4"
 parking_lot = "0.12.0"
 thiserror = "1.0"