Migrate prepare-release-from-ci to new workflow (#20581)

* Migrate prepare-release-from-ci to new workflow

I added a `--releaseChannel (-r)` argument to script. You must choose
either "stable" or "experimental", because every build job now includes
both channels.

The prepare-release-from-npm script is unchanged since those releases
are downloaded from npm, nt CI.

(As a side note, I think we should start preparing semver releases using
the prepare-release-from-ci script, too, and get rid of
prepare-release-from-npm. I think that was a neat idea originally but
because we already run `npm pack` before storing the artifacts in CI,
there's really not much additional safety; the only safeguard it adds is
the requirement that a "next" release must have already been published.)

* Move validation to parse-params module