refactor(release): modernize publication and nightly workflows

[vitormattos]

This PR collects release workflow improvements observed while publishing LibreSign 13.4.5, 14.2.5 and 15.0.7.

Publication action

• remove three skjnldsv/* helper actions that still target Node.js 20;
• remove the remaining set-output warning from the XML helper path;
• keep the existing public inputs/outputs while reading permissions, app metadata and package engines locally.

Release finalization trigger

• ignore pull request closures that only change docs/changelogs/**;
• this prevents history synchronization PRs from creating misleading Finalize release runs while preserving real release preparation PRs, which also change version metadata.

Nightly

• stop building a nightly on every push to every stable branch;
• run once per day (03:17 UTC) or manually;
• resolve the latest stable branch through the existing stable-select action and build that exact branch;
• use the checked-out stable commit when moving the nightly tag and writing release notes;
• remove the same deprecated Node 20 metadata helpers from the nightly path.

This keeps the behavior general: there are no filters by commit message, PR author or release-tool-specific actor.