Bump eslint from 8.57.1 to 9.20.0 in /backend

Bumps [eslint](https://github.com/eslint/eslint) from 8.57.1 to 9.20.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/main/CHANGELOG.md)
- [Commits](eslint/eslint@v8.57.1...v9.20.0)

---
updated-dependencies:
- dependency-name: eslint
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Author: dependabot[bot]