Fix pnpm `ERR_INVALID_THIS` on Node 20 (#51406)
### What?
Fixes pnpm/pnpm#6424
### Why?
So users of Node 20 can contribute to Next.js
### How?
Upgrades pnpm with back compatibility to patch `ERR_INVALID_THIS` error.
If Node 14 support isn't necessary, I'd recommend upgrading to pnpm 8
for continued feature support and committing the lockfile.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
