Update node version for build: v16 -> v20

[d10n]

I'm hoping that this will resolve the error in #2018, which fails with:
ReferenceError: ReadableStream is not defined and

npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: 'whatwg-mimetype@4.0.0',
npm WARN EBADENGINE   required: { node: '>=18' },
npm WARN EBADENGINE   current: { node: 'v16.20.2', npm: '8.19.4' }
npm WARN EBADENGINE }

Will test in the pipeline

---

The pipeline failed, so before we can see if the unsupported engine issue above is resolved, the new pipeline error needs to be fixed.
After investigating a little further, it seems like there might be an issue when checking multiple files for broken links at once, so the 2nd commit checks 1 file at a time as a workaround.

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[iMichaela]

All changes are good. Tested locally.