Bump ruff from 0.6.7 to 0.6.8 in /requirements

Bumps [ruff](https://github.com/astral-sh/ruff) from 0.6.7 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@0.6.7...0.6.8)

---
updated-dependencies:
- dependency-name: ruff
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements/requirements.txt

@@ -5,4 +5,4 @@ cvxpy == 1.5.3
 pytest == 8.3.3 # For unit test
 pytest-xdist == 3.6.1 # For unit test
 mypy == 1.11.2 # For unit test
-ruff == 0.6.7 # For unit test
+ruff == 0.6.8 # For unit test