chore(deps): update actions/upload-artifact action to v6 (#429)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
           config: .htmltest.yaml
 
       - name: Archive htmltest results
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: htmltest-report
           path: tmp/.htmltest/htmltest.log