chore: use os.availableParallelism when available

[merceyz]

What's the problem this PR addresses?

Node.js v19.4.0 comes with a os.availableParallelism function and to quote the docs:

os.cpus().length should not be used to calculate the amount of parallelism available to an application. Use os.availableParallelism() for this purpose.
https://nodejs.org/dist/latest-v19.x/docs/api/os.html#oscpus

How did you fix it?

Use os.availableParallelism when available.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.