Bump futures from 0.3.19 to 0.3.21 (use-ink#424)

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](rust-lang/futures-rs@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>

Cargo.toml

@@ -47,7 +47,7 @@ regex = "1.5.4"
 async-std = { version = "1.10.0", optional = true }
 sp-core = { version = "2.0.1", optional = true }
 subxt = { version = "0.14.0", package = "substrate-subxt", optional = true }
-futures = { version = "0.3.19", optional = true }
+futures = { version = "0.3.21", optional = true }
 hex = { version = "0.4.3", optional = true }
 
 funty = "2.0.0"