Game Engine Upgrade and Quality of Life Improvements

[p]This quality of life upgrade is for long-term stability:[/p]

• [p]Upgraded game engine to Unity 6.3, to keep the game current[/p]
• [p]Removed unused packages and game assets, updated necessary packages (e.g. for camera controls) to optimize game build[/p]
• [p]Upgraded to use Unity's new input system for improved controls[/p]

[p]This makes NinjaThea more maintainable and more accessible. As always, thanks for playing![/p]