Update codecov/codecov-action action to v4 (#415)

* Update codecov/codecov-action action to v4

* Add token

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Dragomir Penev <[email protected]>

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -38,7 +38,9 @@ jobs:
       - name: Run tests
         run: tox run -e unit
       - name: Upload Coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 
   build:
     name: Build charm