Update minimum PNPM to 8.6.8

[kraftbj]

Pretty minor, but there's a fixed CVE associated with the version we're calling in for CI and the Docker build. We can bump to 8.7.* too, but scoped this to just resolving the CVE.

Stumbled upon it during a quick security audit.

https://nvd.nist.gov/vuln/detail/CVE-2023-37478

Proposed changes:

• PNPM from 8.6.2 to 8.6.8 6.8.

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

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Testing instructions:

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[github-actions]

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• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta > Jetpack and enable the update/pnpm-version branch.

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[github-actions]

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[anomiex]

Works for me. I've been using versions up to 8.7.5 locally (as they've been released and I've noticed new versions available) and haven't seen any issues.

[kraftbj]

I've been using versions up to 8.7.5 locally (as they've been released and I've noticed new versions available) and haven't seen any issues.

Same. I figure we could bump it up, but I didn't want to conflate the security impact (however small) with an unrelated set of updates.