bump cgal to 5.6 (#2289) [ci skip]

cmake/recipes/external/cgal.cmake

@@ -7,8 +7,8 @@ message(STATUS "Third-party: creating target 'CGAL::CGAL'")
 include(FetchContent)
 FetchContent_Declare(
     cgal
-    URL https://github.com/CGAL/cgal/releases/download/v5.4/CGAL-5.4-library.tar.xz
-    URL_MD5 996f7ee9ba1553edac60debb115699cd
+    URL https://github.com/CGAL/cgal/releases/download/v5.6/CGAL-5.6-library.tar.xz
+    URL_MD5 793da2d1597f3a5c0e3524f73a0b4039
 )
 FetchContent_GetProperties(cgal)
 if(cgal_POPULATED)