chore: unpin yarn@^4.1.1

[legobeat]

Should be reasonable to allow later yarn 4.x versions.

Related:

• Unvendor Yarn; favor Corepack for installation #243
• ci: run corepack enable before installing dependencies #244

[legobeat]

Does not seem to actually work.

Usage Error: Invalid package manager specification in package.json; expected a semver version

Elsewhere suggested ^[email protected] is not recognized anymore either:

Usage Error: Unsupported package manager specification (^[email protected],^yarn,4.1.1)

[mcmire]

Hmm, lame. Good try though...