Refactor: optimize AbstractShadowRenderer

[capdevon]

The proposed changes in this pull request modernize and clean up the AbstractShadowRenderer class in the jMonkeyEngine project. Key improvements include:

• Updating copyright years.
• Refining JavaDoc comments for clarity.
• Removing unnecessary inheritance (Cloneable).
• Refactoring variable names for consistency and readability (e.g., using geo instead of frustumMdl, replacing loop variables from j to i).
• Improving the frustum debug rendering code for clarity.
• Making the debugfrustums variable protected and adjusting its initialization.
• Enhancing code style by removing redundant or outdated code and warnings.
• Improving overall maintainability and consistency in the class.

These changes do not alter the core logic but improve code quality and documentation.