Set CMake version range to 3.20...4.2.1

CMake support for c++23 was added in 3.20.

Remove statements explicitly setting the following policies to NEW.
These policies were introduced in or before 3.20, and now that 3.20 is
the minimum version they will automatically have the NEW behavior.

Policy:  Introduced in
CMP0079: 3.13
CMP0084: 3.14
CMP0091: 3.15
CMP0092: 3.15
CMP0099: 3.17
CMP0117: 3.20

Disable scanning c++ source files for module imports (introduced as
CMP0155 in 3.28) since we don't use modules and that policy triggers
build errors with Clang if the clang-scan-deps tool isn't installed.

Source/Core/DolphinQt/CMakeLists.txt

@@ -1,8 +1,3 @@
-if(POLICY CMP0084)
-  # Disable trying to search for Qt3/4 if what we actually want is not found
-  cmake_policy(SET CMP0084 NEW)
-endif()
-
 if (MSVC)
   if(_M_ARM_64)
     list(APPEND CMAKE_PREFIX_PATH "${CMAKE_SOURCE_DIR}/Externals/Qt/Qt6.5.1/ARM64")