Update codecov action.

Author: shyuep

.github/workflows/testing.yml

@@ -44,7 +44,7 @@ jobs:
         run: |
           pytest --cov=matgl tests --color=yes
       - name: Upload coverage reports to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         env:
           CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}