Only specify major version of codecov-action

- always give us the latest version of the action in the
  relevant major series
- reduce the number of dependabot PRs to review
- brings consistency with the other actions in this file

.github/workflows/unit-test.yml

@@ -40,4 +40,4 @@ jobs:
       run: coverage run --branch --source=check -m unittest discover --buffer
 
     - name: Upload coverage to Codecov
-      uses: codecov/codecov-action@v4.0.2
+      uses: codecov/codecov-action@v4