task: #3635691 Update Node.js to >= 24.0 and Yarn to 4.18.1

Moves the Node.js pins on 4.0.x to 24, the floor Drupal core main declares in core/package.json. Node.js 20 reached end of life on 30 April 2026.

Issue: https://www.drupal.org/i/3635691

• package.json: engines "node": ">= 24.0" and "yarn": ">= 4.18.1", packageManager yarn@4.18.1, with the Varbase .yarnrc.yml.
• Dependency ranges use carets, ^MAJOR.MINOR.PATCH, as Drupal core's package.json does. No major changes.
• CI: not present.
• DDEV: not present.
• Upsun: not present.
• Lockfiles are no longer committed: yarn.lock removed and ignored in .gitignore.
• Moves from Yarn Classic to Yarn 4.18.1; .yarn/ caches are ignored.

Checked in node:24 (v24.22.2) with no lockfile present: Yarn 4.18.1 install and the webpack theme:build pass. There is no CI on this branch.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release