Add comment in downloads.json to mention that the file is deprecated, and that download-$platform.json should be used instead

downloads.json should not be used, since it's incorrect when not all platforms are on the same version.

I think we can add some comment in the file to indicate that it should not be used, and that download-$platform.json should be used instead.

After a few weeks/months, we can stop generating downloads.json to avoid confusion.