CI: bump first-party Actions to Node-24 majors (checkout, setup-node, setup-python, github-script, cache)

Problem

GitHub is deprecating Node 20 for Actions: runners default to Node 24 on 2026-06-16, and Node 20 is removed in fall 2026. Our workflows pin several first-party actions to Node-20 majors, producing deprecation warnings (e.g. actions/checkout@v4).

A smoke-test (forcing the whole PR pipeline onto Node 24 via FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true) passed end-to-end, so nothing breaks at the cutover — this is cleanup to clear the warnings on a verified runtime.

Change

Bump the clean single-major first-party actions to their verified Node-24 majors, repo-wide:

Deferred (separate, reviewed change): upload-artifact (v4→v6) and download-artifact (v4→v7) are 2–3 major jumps with breaking API history; they run fine on Node 24 when forced, so they're handled separately after reviewing breaking changes.

Note: the first Node-24 release of setup-python, upload-artifact, and download-artifact is not v5 (it's v6/v6/v7 respectively), confirmed from each action's action.yml.

Search the repository's GitHub Actions workflow files for the listed checkout, setup-node, setup-python, github-script, and cache references. Update only those actions to the specified Node-24 majors, leave artifact actions deferred, and verify the workflows with the Node-24 smoke-test pipeline.