Examples: Modernize typedefs to 'using' in hole_filling_example.cpp

[LOTUS9679]

This PR modernizes the C++ code in hole_filling_example.cpp by replacing deprecated typedef declarations with the modern using alias syntax.

Changes:

• Replaced typedef CGAL::... Kernel with using Kernel = ...
• Replaced typedef Mesh::Vertex_handle with using Vertex_handle = ...
• Updated all related type aliases in the file.

Reason:
The using syntax is more readable and consistent with modern C++ standards (C++11 and later) preferred by the CGAL project.

Verification:

• Compiled successfully with Ninja.
• Verified that the example builds without errors.

[sloriot]

Thanks for the proposal but we don't want such changes for now as it should be done globally, and even globally we are not big fan of such changes as it does complicate our life when backporting patches to release branches.