Update remaining GitHub Actions for Node.js 24 (setup-julia, setup-r)

[cderv]

PR #14199 updated all GitHub Actions under our control for Node.js 24 compatibility. Two third-party actions in test-smokes.yml still run on Node.js 20:

• julia-actions/setup-julia@v2 — Node 24 migration merged upstream (Breaking: Migrate from Node 20 to Node 24 julia-actions/setup-julia#374), awaiting v3 release
• r-lib/actions/setup-r@v2 — upstream issue opened (Update actions to use Node.js 24 r-lib/actions#1044), no fix yet
• actions/cache pinned by SHA — used internally by r-lib/actions/setup-r, will resolve when they update

GitHub will force Node.js 24 on June 2, 2026. These are warnings for now.

Track upstream releases and bump in test-smokes.yml when available.