ci: gem release via RubyGems trusted publishing

[ronaldtse]

Summary

• restore gem publishing, repo-correct this time: v* tag (or dispatch) -> gem build -> rubygems/release-gem@v1 with id-token:write, the same RubyGems trusted-publishing pattern secryst 1.0.0 used (no API key secret)
• requires a one-time trusted-publisher link on rubygems.org for interscript-maps (repo interscript/maps, workflow gem-release.yml)

Test plan

• YAML parses
• after merge + rubygems.org trusted publisher configured: dispatch publishes interscript-maps 2.5.0