v1.2.0

ci: make the release job idempotent

Creating the GitHub release from the web UI also creates the tag, which
triggers this workflow, which then tries to create the same release again and
fails with "already_exists". The luarocks upload step is skipped as a result,
so the rock never gets published.

Replace the archived actions/create-release@v1 with
softprops/action-gh-release, which updates an existing release for the tag
instead of failing. It also runs on node24, so the Node 20 deprecation
warning goes away.

Add an explicit contents:write permission since the new action relies on
github.token rather than a passed-in GITHUB_TOKEN.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>