[fix] koa@2.16.0 has wrong version in package.json
## Describe the bug

The package.json for koa 2.16.0 has the version set to "3.0.0-alpha.3", see 
https://github.com/koajs/koa/blob/2.16.0/package.json#L3 

Commit https://github.com/koajs/koa/commit/5054af6e31ffd451a4151a1fe144cef6e5d0d83c looks like a broken backport (which is not merged into the v2.x branch yet), which was not cleaned up all the way for v2.x 

## Actual behavior

The published package does not seem to be correct, see for example https://yarnpkg.com/package?q=koa&name=koa&version=2.16.0&file=%2Fpackage.json, which still contains the unlicensed dependency "only", which was removed in https://github.com/koajs/koa/pull/1824

## Expected behavior

The license field in the package.json matches the tag.

## Checklist

- [x] I have searched through GitHub issues for similar issues.
- [x] I have completely read through the README and documentation.
- [x] I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.
