build(deps): bump flake8 from 6.1.0 to 7.0.0 in /lib/spack/docs (#41971)

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

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
  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>

lib/spack/docs/requirements.txt

@@ -9,5 +9,5 @@ urllib3==2.1.0
 pytest==7.4.4
 isort==5.13.2
 black==23.12.1
-flake8==6.1.0
+flake8==7.0.0
 mypy==1.8.0