doc: modernize code examples in the cluster.md

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, cluster

Description of change

1. var -> let / const
2. concatenate -> template literal
3. == -> ===
4. anonymous function -> arrow function
5. update an obsolete example (see the comment in the issue)
6. simplify cluster.workers iteration: Object.keys().forEach -> for...in

Fixes: https://github.com/nodejs/node/issues/10255