Update actions/upload-artifact action to v6

Author: renovate[bot]

.github/workflows/tests.yml

@@ -53,7 +53,7 @@ jobs:
       - name: Run Playwright tests
         run: CI=1 npm run test
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         if: always()
         with:
           name: test-results