Update CHANGELOG

We do not need to describe changes between prereleases now that the
prerelease phase is over.

Move the main release note draft out of the draft file.

Remove the ROADMAP: we've done everything in it! New ROADMAP to come
once it's ready.

Note that the changeset-publishing code (in
https://github.com/changesets/action/blob/398d7edfb130ad2d4e076a573724f435ea87cea6/src/run.ts#L34)
finds a Markdown section with a heading matching the version (precisely:
no `v`). We can edit the section inside the PR before we merge it, and
that will affect the GitHub Release text.