Migration to upload-artifact version 4

[Murmele]

The problem is that for some reason flatpak-github-actions is using upload-artifact@v3 and then download-artifact@v4 is anymore possible.
The problem came up in this.

https://github.com/actions/download-artifact/blob/main/docs/MIGRATION.md

[AFCMS]

In November 2024 the artifact actions v3 will become unavailable and there is a big workflow warning when using this action, which is annoying.

https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions