fixing https://github.com/advisories/GHSA-f5x3-32g6-xq36

[eggyhead]

We are failing npm validation because of GHSA-f5x3-32g6-xq36. We are updating the dependent package to fix this vulnerability.

This is marked as a breaking change in npm audit:

npm WARN audit Updating lerna to 6.4.1, which is a SemVer major change.

We use lerna for package management. I've run the following scripts to verify this update does not break our usage:

npm run audit-all
npm run bootstrap
npm run build 
npm run clean 
npm run repair 
npm run new-package [test]

I'm also updating the outdated instruction for running new-package so that it is clear for future contributors.