Use codecov-action@v4 (#49)

* Use codecov-action@v4
* Add codecov token

Author: fmauch

.github/workflows/coverage.yaml

@@ -36,9 +36,10 @@ jobs:
         run: |
           catmux_create_session -d catmux/resources/example_session.yaml
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           file: ./coverage.xml
           flags: unittests
           name: codecov-umbrella
           fail_ci_if_error: true
+          token: ${{ secrets.CODECOV_TOKEN }}