Update 13.5 blog post template to give more information about legacy channel

Maybe we can add something explaining what the 13.5 legacy channel is, in the 13.5 blog post template. Maybe linking to this forum post (from tpo/community/support#40163 (closed)): https://forum.torproject.org/t/download-tor-browser-13-5-legacy-on-windows-7-8-and-8-1-and-macos-10-12-10-13-and-10-14/15524