Bump ruff from 0.5.7 to 0.6.8 in /backend (#17)

Bumps [ruff](https://github.com/astral-sh/ruff) from 0.5.7 to 0.6.8.
- [Release notes](https://github.com/astral-sh/ruff/releases)
- [Changelog](https://github.com/astral-sh/ruff/blob/main/CHANGELOG.md)
- [Commits](astral-sh/ruff@0.5.7...0.6.8)

---
updated-dependencies:
- dependency-name: ruff
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/poetry.lock

@@ -43,7 +43,7 @@ slack-bolt = "^1.20.1"
 djlint = "^1.35.2"
 isort = "^5.13.2"
 pre-commit = "^3.8.0"
-ruff = "^0.5.7"
+ruff = "^0.6.8"
 
 [tool.poetry.group.test.dependencies]
 pytest = "^8.3.2"