ci: refactor release workflows into multi-job pipelines with cleanup

[paulkczw]

Summary

Refactors both release workflows (beta + stable) from monolithic single-job workflows into multi-job pipelines with logical units and adds automated cleanup of old pre-releases.

Changes

Multi-Job Pipeline

Both release-beta.yml and release-stable.yml are split into 5 jobs:

1. version — determine version metadata from gradle.properties
2. build — compile, assemble artifacts, create zip
3. sign — generate manifest with SHA-256 hashes + GPG sign
4. publish — publish maven artifacts + create GitHub release
5. cleanup — delete outdated beta/dev pre-releases

Artifacts are passed between jobs via actions/upload-artifact@v4 / actions/download-artifact@v4.

Cleanup Workflow (cleanup-releases.yml)

New reusable workflow that deletes old beta and dev pre-releases (including their tags):

• Triggered as workflow_call from release pipelines or manually via workflow_dispatch
• Reads current version from gradle.properties automatically
• Beta trigger: deletes all beta/dev pre-releases from older versions (e.g. releasing 2.5.1-beta.X deletes 2.5.0-beta.*, 2.4.x-beta.*, etc.)
• Stable trigger: deletes all beta/dev pre-releases of the same version and older (e.g. releasing 2.5.1 deletes 2.5.1-beta.*, 2.5.0-beta.*, etc.)
• Stable releases are never touched

Bugfix

• Fixed GPG signing if condition (env.GPG_PRIVATE_KEY → secrets.GPG_PRIVATE_KEY) — the original condition evaluated before the step-level env was applied, so signing was effectively dead code