Yarn has been added to TYPO3 back in 2017 because
npm shrinkwrap files were not stable and package-lock.json
and `npm ci` not yet established.

By now the yarn version in TYPO3 (v1) got outdated
and an update to yarn v3 would be required sooner
or later. The update to yarn v3 brings quite some
problems for the TYPO3 codebase:

 - Yarn v3 is executed/proxied through yarn v1
   (installed on the developer machine) – therefore the
   yarn v3 executable needs to be tracked within git.
   See https://yarnpkg.com/cli/set/version#details

 - Yarn v3 pulls in @types/node (through the "got" package) for
   unknown reasons, causing clashes with TypeScript declarations
   for requirejs (window.require vs node's require())

As package-lock.json is solid thesedays, there is no real
need to depend on yarn anymore. We basically switch
to npm because that switch is way more straightforward than
migrating to yarn v3 and it removes one dependency.

Note that `npm install` properly honors `...