build(deps): bump futures-util in /atcoder-problems-backend

Bumps [futures-util](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-util
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

atcoder-problems-backend/Cargo.lock

@@ -31,7 +31,7 @@ reqwest = { version = "0.11", features = ["json"] }
 async-trait = "0.1"
 
 anyhow = "1.0"
-futures-util = "0.3.19"
+futures-util = "0.3.21"
 
 [dev-dependencies]
 httpmock = "0.6.6"