ci: bump setup-node to v4 and Node to 20 in release.yml #1198

Description

.github/workflows/release.yml still pins actions/setup-node@v3 with node-version: 18. Both are behind the rest of the repo and carry real costs:

• actions/setup-node@v3 depends on the Node 16 Actions runtime, which GitHub has deprecated. @v4 runs on Node 20.
• Node.js 18 reached end-of-life on 2025-04-30 and no longer receives security updates. Node 20 is Active LTS.
• This is the workflow that runs semantic-release to cut every spec release, so the toolchain it runs on is particularly sensitive.

Sister workflows in this repo already use the modern versions:

• .github/workflows/validate-examples.yml — setup-node@v4, node-version: '20'
• .github/workflows/validate-embedded-examples.yml — setup-node@v4, node-version: '20'

This issue tracks aligning release.yml with them.

Proposed change

In .github/workflows/release.yml:

• actions/setup-node@v3 → actions/setup-node@v4
• node-version: 18 → node-version: 20

semantic-release@21.0.7 (the version pinned in the workflow) and conventional-changelog-conventionalcommits@6.1.0 both support Node 20, so this is a drop-in bump.

Scope

• 2 line edits in 1 file. No other workflow or script touched.
• Complements ci: bump actions/checkout from v3 to v4 in local workflows #1195 (checkout v3 -> v4) and ci: bump actions/github-script from v6 to v7 in local workflows #1197 (github-script v6 -> v7), finishing the move of the local workflows off deprecated Actions runtimes.