Repair wp/6.0 branch Actions: Test against a single Node.js version - #84268

Part of #82933.

What?

Removes the node matrix dimension from the two jobs on wp/6.0 that still test against multiple Node.js versions, and standardizes how every actions/setup-node call on this branch sources its Node.js version:

• create-block.yml's checks job: removes node: [14] and its include: - node: 12 entry from the matrix, leaving only the os dimension.
• unit-test.yml's unit-js job: removes node: ['12', '14'] and the now-empty strategy block along with it.
• bundle-size.yml's build, end2end-test.yml's admin and end2end-test-playwright.yml's e2e jobs: removes the leftover single-value node: ['14'] matrix entries, which nothing reads anymore, along with the strategy blocks they leave with nothing to configure.
• Every remaining actions/setup-node call across .github/workflows/ (whether it previously hardcoded node-version: 14 or read it from a single-value node matrix) now uses node-version-file: .nvmrc instead of an explicit node-version input.

Why?

This branch is frozen and will never need to verify compatibility across multiple Node.js versions again, so running the same tests twice against different Node.js versions only spends extra CI time without adding any coverage that matters. Standardizing every workflow on node-version-file: .nvmrc removes the redundant, easy-to-miss duplication of the Node.js version number across seven workflow steps, so the version used in CI always matches the one the rest of the toolchain (nvm, local development) already reads from .nvmrc.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.