Bump actions/upload-artifact from 5 to 6 (#379)

Author: dependabot[bot]

.github/workflows/gh-pages.yaml

@@ -48,7 +48,7 @@ jobs:
           echo "::remove-matcher owner=linkchecker::"
       - name: Archive documentation
         if: ${{ github.event_name == 'pull_request' }}
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: documentation
           path: doc/