This is my first attempt at contributing to Lichess, feedback is welcome 😄

What this PR changes

• Specifies the used pnpm version in the packageManager field which is used by corepack
• Removes the version from the GitHub worklows, as pnpm/ctions-setup uses the exact version specified in packageManager
• Switches from pnpm v8 to v9 and node from v14 to v20 (because of corepack and pnpm)
• Upgrades the pnpm-lock.yaml to the latest format without changing any version (I just ran pnpm install locally)

How to update this PR (if it was sitting stale for a while

• rebase from latest master, taking the pnpm-lock.yaml from the current master
• update to latest pnpm: corepack use pnpm@latest
• migrate pnpm-lock.yaml to latest format: pnpm install

Open tasks

If this is merged, the Lichess Development Onboarding must be updated, I recommend to use the latest LST (e.g. 20) and run corepack enable once so you'll always have the correct version of pnpm, which is also described on the pnpm documentation.