Bump minimum CMake version to 3.20.

Author: kblaschke

CMakeLists.txt

@@ -1,10 +1,4 @@
-cmake_minimum_required(VERSION 3.15 FATAL_ERROR)
-
-if(CMAKE_VERSION VERSION_LESS 3.19 AND CMAKE_GENERATOR STREQUAL "Xcode")
-    message(AUTHOR_WARNING "Using a CMake version before 3.19 with a recent Xcode SDK and the Xcode generator "
-            "will likely result in CMake failing to find the AppleClang compiler. Either upgrade CMake to at least "
-            "version 3.19 or use a different generator, e.g. \"Unix Makefiles\" or \"Ninja\".")
-endif()
+cmake_minimum_required(VERSION 3.20 FATAL_ERROR)
 
 include(CMakeDependentOption)
 include(CheckSymbolExists)