chore(deps-dev): bump flake8 from 6.1.0 to 7.0.0 in /backend (#900)

Bumps [flake8](https://github.com/pycqa/flake8) from 6.1.0 to 7.0.0.
- [Commits](PyCQA/flake8@6.1.0...7.0.0)

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

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

backend/dev-requirements.txt

@@ -2,7 +2,7 @@
 bandit==1.7.8
 black==24.3.0
 coverage==7.3.1
-flake8==6.1.0
+flake8==7.0.0
 isort==5.12.0
 mypy-protobuf==3.5.0
 parameterized==0.9.0