Add automatic GitLab release/milestone detection and robust configuration support

This MR introduces automatic GitLab release detection and several major improvements to enhance the bot's functionality and usability.

Key Features

🚀 Automatic GitLab Release Detection

• Implements logic to auto-detect the current GitLab release/milestone
• Based on the rule that releases occur on the third Thursday of each month
• Minor versions increment monthly and roll over to the next major after .11
• Base release is 18.0 (May 2025)
• Falls back to detected release when milestoneFilter is empty

📅 Dynamic Message Date

• Automatically replaces <<MERGE_DATE>> placeholder in message.md
• Uses the correct merge date (Monday before the release)

⚙️ Flexible Configuration

• Support for multiple configuration sources:
  • Config file
  • Environment variables
  • Command-line arguments
• CLI arguments take precedence over other sources

🔧 Enhanced Developer Experience

• Improved error handling and logging for easier debugging
• Comprehensive documentation updates in README
• Added type hints throughout the codebase
• Better user experience with clearer error messages

✅ Testing & CI/CD

• Comprehensive test coverage for:
  • Release calculation logic
  • Merge date calculations
  • Configuration handling
• Added suggestions and support for scheduled GitLab CI/CD runs

How to test

1. Run the script without milestoneFilter to verify auto-detection
2. Test with different configuration methods (file, env vars, CLI)
3. Verify <<MERGE_DATE>> replacement in message output
4. Check error handling with invalid inputs
5. Run the test suite: python -m pytest

Author's checklist

• Tests added for new functionality
• Documentation updated (README)
• Type hints added
• Error handling improved
• CI/CD integration documented