chore: un-bump codecov/codecov-action from 5 to 4 (#32314)

Reverts #32173

Codecov v5 is throwing errors with some PRs (e.g. [failing workflow](https://github.com/aws/aws-cdk/actions/runs/12012047131/job/33482380518)) and reportedly has many issues. The quick-fix until they resolve their issues is to downgrade the version of codecov we are using back to v4.

Author: Leo10Gama

.github/workflows/codecov.yml

@@ -28,7 +28,7 @@ jobs:
         run: cd packages/aws-cdk && yarn test
 
       - name: Upload results to Codecov
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v4
         with:
           directory: packages/aws-cdk/coverage
           fail_ci_if_error: true