CMake 4.0 FetchContent warning

[Robadob]

CMake Warning (dev) at C:/Program Files/CMake/share/cmake-4.0/Modules/FetchContent.cmake:1953 (message):
  Calling FetchContent_Populate(flamegpu2) is deprecated, call
  FetchContent_MakeAvailable(flamegpu2) instead.  Policy CMP0169 can be set
  to OLD to allow FetchContent_Populate(flamegpu2) to be called directly for
  now, but the ability to call it with declared details will be removed
  completely in a future version.
Call Stack (most recent call first):
  cmake/flamegpu2.cmake:65 (FetchContent_Populate)
  CMakeLists.txt:13 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

[Robadob]

Also this is still pulling RC2, so fails bc of rapidjson.

[ptheywood]

This will need to be fixed as part of #20 (though probably skipping to rc.4).