updating gh 'actions/cache' away from deprecated v2

[melange396]

actions/cache@v2 is now deprecated, this sets it to v4 in all of the references to it in this repo.

this is the gh actions failure that prompted this change: https://github.com/cmu-delphi/delphi-epidata/actions/runs/13397502938/job/37420098843

here is a subsequent successful run of it (run from this branch) : https://github.com/cmu-delphi/delphi-epidata/actions/runs/13530875353/job/37812545923
and the PR it produced: #1605

[melange396]

whoops! i force-pushed because this PR branch was not based on the current dev branch, but instead on an older one which did not include #1548. in a moment, i will re-run the csv sync gh action and update the links in the first comment of this PR with new references.

[korlaxxalrok]

LGTM 👍

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[melange396]

whoops again! #1605 was merged into this branch instead of the dev branch because i ran the sync workflow from here (and i forgot to change it in the other PR). i think we should be able to remedy this (and keep the commit history sane) by doing a "rebase merge" on this PR...