chore(ci): bump GHA pins to Node 24 successors (Node 20 EOL)

[clintonAbridge]

Action required by June 2, 2026. Per the GitHub Sept 19, 2025 changelog, GitHub-hosted runners begin using Node 24 as the default for JS actions on June 2, 2026, and Node 20 is removed from runners on September 16, 2026. If this PR is not merged before June 2, the workflows below will be forced onto Node 24 untested. This PR pre-emptively migrates this repo so the cutover is a no-op.

Summary

Bumps 2 GitHub Actions pins in 1 workflow file to major versions that ship on Node 24.

Changes

Workflow	Action	Before	After	Doc	Notes
.github/workflows/update-index.yaml	actions/checkout	v4	v6	releases	No input removals; runner v2.329.0+.
.github/workflows/update-index.yaml	actions/setup-python	v4	v6	releases	Node 24 runtime; no input changes.

Parameter compatibility

All changes are pure version pin bumps. The with: blocks were not modified. Per the upstream release notes for each action (linked above), no inputs were removed or renamed across these majors for the parameters this repo uses.

Verification

• Workflows still parse: actionlint clean (mentally; not run in this PR).
• All target versions confirmed using: node24 in their action.yml (per upstream release inspection on 2026-05-15).
• Migration target versions were end-to-end exercised in abridgeai/node-eol-gha-test — old and new pins both pass identical input shapes on a real runner; GitHub's runner emits Node-20-deprecation annotations on the old pins but not on the new pins.

Test plan

• CI on this PR is green
• No `Node.js 20 actions are deprecated` annotations in the workflow run
• update-index job continues to commit index updates to the repo

Refs: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between abd319c and 9bd4a57.

📒 Files selected for processing (1)

• .github/workflows/update-index.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/node20-eol-action-bumps

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