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

Bumps [serde_json](https://github.com/serde-rs/json) from 1.0.74 to 1.0.81.
- [Release notes](https://github.com/serde-rs/json/releases)
- [Commits](serde-rs/json@v1.0.74...v1.0.81)

---
updated-dependencies:
- dependency-name: serde_json
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]