fix: Debian changelog generation for Ubuntu PPA builds

[inureyes]

Summary

Fix changelog generation script and update changelog for Ubuntu PPA/Launchpad builds.

Changes

• Fix date command compatibility: Add cross-platform support for date formatting (GNU date on Linux, BSD date on macOS)
• Update changelog content: Include proper release notes from v0.6.1 with the rebrand from 'Backend.AI SSH' to 'Broadcast SSH'
• Improve script robustness: Handle both Linux and macOS environments in the changelog update script

Technical Details

The debian/update-changelog.sh script now detects the system's date command type and uses appropriate syntax:

• GNU date (Linux): date -d "$DATE"
• BSD date (macOS): date -j -f "%Y-%m-%dT%H:%M:%SZ" "$DATE"

This ensures the changelog can be generated correctly on both development machines (macOS) and CI/CD environments (Linux).

Testing

• Script tested on both macOS and Linux environments
• Changelog format validated for Debian/Ubuntu packaging standards
• Date formatting verified for both GNU and BSD date commands