Move CI and release workflows off deprecated Node.js 20 #27

Problem

GitHub Actions warns on every run (first seen on #25):

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-node@v4.

Separately, the workflows install Node 20 for our own steps:

.github/workflows/ci.yml:76       node-version: 20   (tests job)
.github/workflows/ci.yml:104      node-version: 20   (js job)
.github/workflows/release.yml:20  node-version: 20

Node 20 reached end of life in April 2026. It has already bitten us once: node --test with a glob pattern needs Node 21+, which is why test:unit had to change in #25 (87c4761).

Fix

• Move all three node-version settings to the current LTS (22 or 24), and keep them in sync. One option is an .nvmrc read via node-version-file: .nvmrc, which also pins local development.
• Move actions/checkout, actions/setup-node and actions/cache to the major versions that run on Node 24, if they're released. Keep shivammathur/setup-php@v2 and softprops/action-gh-release@v2 current too.
• Consider an engines.node field in package.json so a too-old local Node fails clearly.
• Coordinate with Clear npm audit findings in dev dependencies (@wordpress/scripts tree) #26: the @wordpress/scripts upgrade may raise the minimum Node version.

Acceptance

• CI runs with no Node.js deprecation warning.
• A test release (vX.Y.Z-rc.N) still builds and attaches jc-core-functionality.zip, published as a prerelease.