MNT: upgrade upload/download-artifact GitHub actions

[neutrinoceros]

Hi, version 4 of upload-artifact and download-artifact were recently released.
Staying on old version is not viable long term because GitHub will sooner or later drop support for the old versions of node that these depend on.
Since migration to v4 is not as straightfoward as bumping the version number, I'm helping all astropy-related packages byt manually performing the necessary changes.

(in this specific case it is actually trivial, I'm just doing them all at once so no reason to skip this repo)

• Check the box to confirm that you are familiar with the contributing guidelines and/or indicate (check the box) that you are familiar with our contributing workflow.
• [N/A] Confirm that any contributed tutorials contain a complete Introduction which includes an Author list, Learning Goals, Keywords, Companion Content (if applicable), and a Summary.
• Check the box to confirm that you are familiar with the Astropy community code of conduct and you agree to follow the CoC.