chore(deps-dev): bump flake8 from 5.0.4 to 6.0.0 in /backend

Bumps [flake8](https://github.com/pycqa/flake8) from 5.0.4 to 6.0.0.
- [Commits](PyCQA/flake8@5.0.4...6.0.0)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

backend/dev-requirements.txt

@@ -2,7 +2,7 @@
 bandit==1.7.4
 black==23.3.0
 coverage==6.4.4
-flake8==5.0.4
+flake8==6.0.0
 isort==5.12.0
 mypy-protobuf==3.4.0
 parameterized==0.8.1