Update CMake minimum version to 3.26 (#2235)

When working on #2226 I inadvertently introduce the use of `copy_directory_if_different` which doesn't exist until CMake 3.26.

Author: ld-kerley

CMakeLists.txt

@@ -5,7 +5,7 @@ set(MATERIALX_BUILD_VERSION 3)
 set(MATERIALX_LIBRARY_VERSION ${MATERIALX_MAJOR_VERSION}.${MATERIALX_MINOR_VERSION}.${MATERIALX_BUILD_VERSION})
 
 # Cmake setup
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.26)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
 set(CMAKE_MACOSX_RPATH ON)