Breaking change: Remove CMake submodule support in favor of fetched or installed dependencies.

This flips the default behavior to "fetch", downloading local copies of required dependencies.  This can be disabled by setting `-Dprotobuf_FETCH_DEPENDENCIES=OFF`, in which case we will look for a local installation using find_package.  Setting `-Dprotobuf_ABSL_PROVIDER=package` will continue to have the same behavior as before.

See https://protobuf.dev/news/2024-10-02/#replace-cmake-submods for more details.

#test-continuous

PiperOrigin-RevId: 686649864