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/bin/node/testing/Cargo.toml

@@ -48,4 +48,4 @@ sp-blockchain = { version = "4.0.0-dev", path = "../../../primitives/blockchain"
 log = "0.4.8"
 tempfile = "3.1.0"
 fs_extra = "1"
-futures = "0.3.19"
+futures = "0.3.21"