docs: Update pnpm supported versions

[Spice-Z]

Why:

pnpm v9 is supported from this PR. https://github.com/dependabot/dependabot-core/pull/9668

But the document doesn't include v9, so I added it.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Before	After

Check off the following:

• [x] I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

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• [x] For content changes, I have completed the self-review checklist.