`lerna` is a bit stale, and as such it currently has some outdated
dependencies which Dependabot warns on.
Fortunately, npm 7 supports monorepos natively, via "workspaces". So
simply replace `lerna` with this feature. The migration is pretty
neat and requires a very little invervention.
Because Node.js 14 comes with npm 6 by default, upgrade it manually
as shown in https://github.com/bahmutov/npm-install/issues/103#issuecomment-931226602.
Nathan Adams