Use yarn v4

[heyLu]

Tried instructions at https://yarnpkg.com/getting-started/install, but yarn init -2 resets the package.json, so I reverted that and only left the .yarn stuff in place.

[changeset-bot]

⚠️ No Changeset found

Latest commit: fe01814

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[heyLu]

Same error here, also locally. The error was initially different, complaining it could not find esling-plugin-jest. After installing that locally using yarn add --dev eslint-plugin-jest@latest the same error appears as in #125.

[heyLu]

Did not help, closing. FYI: The correct migration steps to modern yarn seem to be at https://yarnpkg.com/migration/guide (because we have an existing project).