Update default node version to 18

[byrond]

Updates the default Node.js version for new projects created from this skeleton to 18.

Testing

• Follow the steps in the readme to create a new project using this branch (update-to-node-18). For the Composer command, use:

create-project palantirnet/drupal-skeleton example dev-update-to-node-18 --no-interaction

• ddev . node --version
• Verify Node 18 is used in the DDEV web container.

[jesconstantine]

This worked for me!