Use npm v8 instead of v9 on CI testing

[ybiquitous]

Which issue, if any, is this issue related to?

I believe this PR should fix the CI failures with PR #6463.

Is there anything in the PR that needs further explanation?

See the announcement about npm v9:
https://github.blog/changelog/2022-11-09-npm-cli-v9-is-now-generally-available/

See the failed job:
https://github.com/stylelint/stylelint/actions/runs/3433111899

[changeset-bot]

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[ybiquitous]

CI passes! This PR is blocking other PRs, so I'll merge it.