Only publish updated packages from CI

[shawnbot]

Right now our CI builds take longer than they need to because we're defensively publishing new canary or release candidate versions of every module to npm. This caused some problems when we started maintaining multiple release branches (see #350) which I attempted to work around with commits: a11cdd0, 58f6a8f, 9156588, fe7aa58, f1aec75. But, assuming that problem is fixed, it's really just a bandaid because we could shave a lot of time and "effort" off the CI builds by only publishing new versions (and updating the corresponding package.json files) for modules that have been updated since the most recent release.

Some test cases:

1. If we make a change to primer-utilities, only that module and the modules that depend on it (directly or indirectly) should be bumped.
2. If we make a change to primer-support, all of the modules should be bumped because they rely on it either directly or indirectly.

[shawnbot]

In theory, Lerna should do this correctly. Ideally, publish --canary would only bump versions of packages that have been "updated" according to the algorithm that drives lerna updated.

For release candidates, we should be able to do:

lerna publish --cd-version prerelease --preid rc

But I think that we may end up with some collisions in that case, too. For instance, if two release branches have the same version of a package that isn't being updated, they would both try to publish the same release candidate and fail. I guess in that case it wouldn't be an error since the first one would "win", assuming the version hasn't changed? 🤔

[shawnbot]

Judging from status checks for #607, this is working as expected for release candidates. Slimming canary releases feels like a nice-to-have to me. 🤷‍♂️

[shawnbot]

This is the case as of #625, which includes some release script simplifications. AFAIK, canary releases bump only the updated packages.