[6.15.z] Bump ruff from 0.6.7 to 0.6.8 (#16521)

requirements-optional.txt

@@ -2,7 +2,7 @@
 pytest-cov==5.0.0
 redis==5.0.8
 pre-commit==3.8.0
-ruff==0.6.7
+ruff==0.6.8
 
 # For generating documentation.
 sphinx==8.0.2