Pin pnpm version using corepack

[RobinVdBroeck]

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I had pnpm 9 (the latest version) installed globally, which resulted in changes to package-lock.json when running pnpm install. This pins the version of pnpm to pnpm v8 when using corepack. When not using corepack, it will still use the global installed version.

I've updated the documentation about this in CONTRIBUTING.md, let me know if I should also add this information in another place.