update actions/upload-artifact@v4

.github/workflows/nbval.yaml

@@ -36,7 +36,7 @@ jobs:
         mkdir docs/artifacts
         jupyter nbconvert --to html --execute --ExecutePreprocessor.timeout=600 --allow-errors --output artifacts/${{ matrix.notebook-file }}.html docs/${{ matrix.notebook-file }}
     - name: Upload artifacts
-      uses: actions/upload-artifact@v2
+      uses: actions/upload-artifact@v4
       with:
         name: executed_notebooks
         path: docs/artifacts