cmake: Use C++20 for ImGui

We use Dolphin code (specifically Common's ASSERT) from ImGui, and that will use C++20 features in the next commit, so ImGui needs to be built with C++20 for that to work properly.

Externals/imgui/CMakeLists.txt

@@ -1,5 +1,5 @@
 if (NOT MSVC)
-  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD 20)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
   set(CMAKE_CXX_EXTENSIONS OFF)
 endif()