CI: GitHub Actions Node.js 20 deprecation

[bilby91]

GitHub deprecated Node.js 20 actions; warning currently surfaces on every CI run:

Node.js 20 actions are deprecated. The following actions are running on
Node.js 20 and may not work as expected: actions/checkout@v4,
actions/setup-go@v5. Actions will be forced to run with Node.js 24 by
default starting June 2nd, 2026.

Bump to current versions when available. As a stopgap we can set
FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true on the runner. Don't
delay v0.1.0 over this; just track.