ci/release: implement trusted publishing

Completely rewrite the release workflow. There are a couple key
differences now:
* Releases are triggered by GitHub releases; which can refer to an
  existing tag, or create one on the fly.
* Credentials are no longer needed to perform a release, instead we use
  the PyPI trusted publishing flow.
* The build and upload tasks are split to ensure the credentials are
  safeguarded.
* Uploads require approval from one other maintainer (beyond the person
  who triggered the build).

Note that PyPI will only accept uploads from the 'pypi' environment,
triggered in this workflow file.