Check and potentially bump CI/CD actions. #17

There is an issue with sigstore/gh-action-sigstore-python@v2, sigstore/gh-action-sigstore-python@v3 works

And we seem to be missing the contents: write permission (which was never before required), update to softprops/action-gh-release@v2 while we're at it.

prepare_release.yml

...
  create-release:
    needs:
      - build
    runs-on: ubuntu-latest
    permissions:
      contents: write
...