Change upload-artifact and download-artifact version

[alexgarel]

I got a message from github about a deprecation and I saw that this is a repository using it.

See .github/workflows/postsubmit.yml

Message from Github:

Artifact actions v3 will be deprecated by December 5, 2024. You are receiving this email because you have GitHub Actions workflows using v3 of actions/upload-artifact or actions/download-artifact. After this date using v3 of these actions will result in a workflow failure. Artifacts within their retention period will remain accessible from the UI or REST API regardless of the version used to upload.

To raise awareness of the upcoming removal, we will temporarily fail jobs using v3 of actions/upload-artifact or actions/download-artifact. Builds that are scheduled to run during the brownout periods will fail. The brownouts are scheduled for the following dates and times: