Revert Zammad: use Debian 12 and dynamic APT source version

[MickLesk]

✍️ Description

Edit; Zammad has removed the deb13 stable repo to alpha, so i switched back to debian 12.

Old PR-Description;

- Set default to Debian 13 for stable Zammad support
- Make APT source use dynamic VERSION_ID (from tools.func) instead of hardcoded value, This will automatically work when Zammad 7.0 stable releases with Debian 13 support.

🔗 Related Issue

Fixes #9725

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.