Update codecov action to v4

The documentation is very confusing, but it seems to me that the new
version supports tokenless uploads in a better way.

It says everywhere that a token is really required, but then it also
says that PRs from forks are an exception to that (but that is
how it has been in the past anyway).

So let's see if this helps.

https://docs.codecov.com/docs/codecov-uploader#supporting-tokenless-uploads

https://community.codecov.com/t/upload-issues-unable-to-locate-build-via-github-actions-api/3954/22
codecov/feedback#126
codecov/engineering-team#665
https://github.com/codecov/codecov-action

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Static checks, unit- and integration tests
         run: tools/container_run_ci
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           # should not be necessary for public repos, but might help avoid sporadic upload token errors
           token: ${{ secrets.CODECOV_TOKEN }}