Change CI to use download-artifact@v4

[JacksonBurns]

The GitHub official upload/download action now supports retrieving from previous runs and cross-repository downloads (for RMG-database), would be good to switch the CI to use this instead of the third party action we use now.

[JacksonBurns]

Also see: https://github.blog/2024-02-12-get-started-with-v4-of-github-actions-artifacts/

[github-actions]

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