Simplify keyring management and fix key verification

- Move GPG key to /etc/apt/trusted.gpg.d/guideos.asc
- Use ASCII-armored key format for better compatibility
- Remove obsolete keyring files from /usr/share/keyrings/
- Update sources.list.d to reference new key location
- Add postinst script to clean up old configurations
- Add postrm script to properly purge configuration files
- Update to version 1.0-3
