migrate from yarn to pnpm

[NickyMeuleman]

Yarn1 causes issues and is no longer well maintained.
So no bugfix seems to be coming: yarnpkg/yarn#7734 (comment)

The specific issue this repo has: Unable to install any packages with yarn add or yarn upgrade-interactive.
This is worked around by downgrading yarn with yarn policies set-version 1.19.1

Goal: move repo to pnpm with turborepo.
This will probably need some specific gatsby packages to make it work as I don't think gatsby works with pnpm out of the box