Let's upgrade Lerna to version 7.1.4. This will keep us in sync with JupyterLab, make the future Yarn 3/jlpm migration smoother, and give us the latest workspace and monorepo improvements from Lerna 7.x.

What’s the plan?

• Update lerna in our devDependencies in package.json to ^7.1.4
• Run yarn install (or npm install) to refresh the lockfile
• Update package.json to replace usage of lerna boostrap with yarn install
• Double-check lerna.json for any compatibility tweaks
• Try out the main Lerna commands (run build, run test, clean)
• Update docs (README, start a CONTRIBUTING doc?) if any Lerna usage or commands have changed