task: #3635741 Update Node.js to >= 24.0 and Yarn to 4.18.1

Moves the Node.js pins on 2.1.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/3635741

• package.json: engines node >= 24.0. tests/package.json too.
• Yarn 4.18.1: packageManager yarn@4.18.1, engines yarn >= 4.18.1, the Varbase .yarnrc.yml.
• .gitlab-ci.yml: .composer-base and stylelint read .yarnrc.core.yml, so Drupal core's Yarn 4.9.1 skips the project .yarnrc.yml.
• .gitlab-ci.yml: DRUPAL_CORE 11.4.11.
• Remove yarn.lock and ignore JavaScript lockfiles.
• Use caret version ranges, as Drupal core does.
• phpstan.neon: the yarn.lock exclude is marked optional ((?)), because phpstan rejects an exclude path that does not exist.

Checked in node:24 (v24.22.2) with no lockfile present: Yarn 4.18.1 install and tests/ npm install pass. MR pipeline 1033888 is green.

Remaining tasks

• The tests/ dry run fails with @cucumber/pretty-formatter does not export a function/class. Pre-existing: the base branch fails the same under node:22 with its lockfile.

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