CMake recommends FetchContent_MakeAvailable over FetchContent_Populate

[LandryNorris]

According to the CMake docs, FetchContent_Populate is deprecated as of 3.30. When syncing the examples, the following is printed by CMake 3.30:

  Calling FetchContent_Populate(pico_sdk) is deprecated, call
  FetchContent_MakeAvailable(pico_sdk) instead.  Policy CMP0169 can be set to
  OLD to allow FetchContent_Populate(pico_sdk) to be called directly for now,
  but the ability to call it with declared details will be removed completely
  in a future version.

FetchContent_MakeAvailable was added in 3.14, so to maintain compatibility with 3.13, a version check would be needed.

[LandryNorris]

Currently there are two usages of FetchContent_Populate according to a search. I changed the usage in the import script when copying it into my project, and had no issues (the if check can also be removed, since MakeAvailable handles whether the content has already been populated).

[will-v-pi]

Neither of these can simply be changed to MakeAvailable, as they both need to populate the content without adding it to the build. I will look at changing the calls to use the other signature for FetchContent_Populate which hasn't been deprecated, so we can get the same behaviour. I believe the other signature has been around since FetchContent_Populate was added in 3.11, so no version check will be necessary.

[kilograham]

merged into develop