ci: automate 1.8 LuaRock releases

[shreemaan-abhishek]

Summary

• publish API7-maintained rockspecs when they land on release/1.8
• create the matching GitHub release before uploading to LuaRocks
• validate the release commit message, rockspec path, and LuaRocks credentials

Checklist

• Tests pass with docker build -f tests/Dockerfile . -t lua-resty-openidc/test
• Tests pass with docker run -t --rm lua-resty-openidc/test:latest
• ChangeLog is updated

Both complete CI runs pass. A ChangeLog update is not applicable because this PR only adds the publishing workflow; the workflow YAML was also parsed locally.

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