build(deps): bump rand from 0.7.3 to 0.8.4 in /atcoder-problems-backend

Bumps [rand](https://github.com/rust-random/rand) from 0.7.3 to 0.8.4.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@0.7.3...0.8.4)

---
updated-dependencies:
- dependency-name: rand
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

atcoder-problems-backend/Cargo.lock

@@ -10,7 +10,7 @@ publish = false
 log = "0.4"
 fern = "0.6.1"
 
-rand = "0.7.3"
+rand = "0.8.4"
 chrono = "0.4"
 regex = "1"
 rust-s3 = { version = "0.27.0-rc4", features = ["no-verify-ssl"] }