Bump ruff from 0.6.7 to 0.6.8

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] <support@github.com>

requirements.txt

@@ -9,7 +9,7 @@ statsmodels==0.14.3
 pytest==8.3.3
 pytest-cov==5.0.0
 zarr==2.18.3
-ruff==0.6.7
+ruff==0.6.8
 netcdf4==1.7.0
 h5netcdf==1.3.0
 impactlab-tools==0.6.0