ci: move to the Node 24 majors of checkout, setup-node and cache

[bendersej]

Background

Every run warns that actions/checkout@v4, actions/setup-node@v4 and actions/cache@v4 target Node.js 20, which GitHub is removing from its runners (https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/).

Changes

• actions/checkout v4 → v7, actions/setup-node v4 → v7, actions/cache v4 → v6, the current majors, all on Node.js 24

Notes

• Breaking changes reviewed: checkout v6 keeps its credentials in a separate file (git push from workflows still works), checkout v7 blocks fork checkouts under pull_request_target / workflow_run (not used here); setup-node v6 limits automatic caching to npm, and v7 drops a placeholder NODE_AUTH_TOKEN, which is what npm trusted publishing (OIDC, used by the Release workflow) expects; cache v6 only moves to ESM
• The npm publish path is only exercised by the next SDK release: watch the first Release run that publishes
• The "ubuntu-latest will migrate to Ubuntu 26" notice needs no change: the jobs use apt, Docker and Node only