Bump futures from 0.3.14 to 0.3.15 (#3178)

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.3.14 to 0.3.15.
- [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/0.3.14...0.3.15)

---
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>

polkadot/node/subsystem-util/Cargo.toml

@@ -7,7 +7,7 @@ description = "Subsystem traits and message definitions"
 
 [dependencies]
 async-trait = "0.1.42"
-futures = "0.3.12"
+futures = "0.3.15"
 futures-timer = "3.0.2"
 parity-scale-codec = { version = "2.0.0", default-features = false, features = ["derive"] }
 parking_lot = { version = "0.11.1", optional = true }
@@ -35,7 +35,7 @@ substrate-prometheus-endpoint = { git = "https://github.com/paritytech/substrate
 assert_matches = "1.4.0"
 async-trait = "0.1.42"
 env_logger = "0.8.2"
-futures = { version = "0.3.12", features = ["thread-pool"] }
+futures = { version = "0.3.15", features = ["thread-pool"] }
 log = "0.4.13"
 parking_lot = "0.11.1"
 polkadot-node-subsystem-test-helpers = { path = "../subsystem-test-helpers" }