Describe

The popular https://github.com/actions/upload-artifact & https://github.com/actions/download-artifact recently got a major update to v4, which is incompatible with v3 actions.
The v4 release offers a considerable boost in the efficiency of artifacts' uploads & downloads, so it's generally a much desired bump.
By the look of things, this breaks the flow for using the test-reporter with public repositories, as test-reporter uses those with v3.

Example of a bump which seems to break the test reports flow (Scala CLI repo)

I did not do in-depth research on this topic, but it seems that bumping to v4 in the Scala CLI repo breaks the test reports:

• Bump actions/upload-artifact & actions/download-artifact from 3 to 4 VirtusLab/scala-cli#2701

We use the public repo setup like this:
https://github.com/VirtusLab/scala-cli/blob/3dcfb3357d9c35bd71be27548e7e4e82fc6ef528/.github/workflows/test-report.yml
So we generate test reports with Mill, use our custom script to change them into the JUnit XML format and then upload the artifacts for the test report workflow to pick it up, nothing special. Should be seen as the standard java-junit flow for a public repo.

Proposed solution

Bump test-reporter actions to v4 & probably release a v2 version of the test-reporter action, to prevent breaking old workflows and let them bump their stuff in peace.

Alternatives considered

Stay at v3, I guess?