Don't rely on RecommendedTBBVersions to create download links

Over at #31307 (moved), we were dealing with a broken GetTor because it made assumptions on the list length available at https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions/. See this comment for more details.

While our emergency fix worked, boklm mentions on IRC that RecommendedTBBVersions is not a good way to learn about the current Tor Browser version:

 20:46  boklm│ RecommendedTBBVersions is not a good way to get the current Tor Browser version
 20:47  boklm│ https://aus1.torproject.org/torbrowser/update_3/release/downloads.json is better
 20:48  boklm│ for example with: wget -q -O - https://aus1.torproject.org/torbrowser/update_3/release/downloads.json | jq -r .version

Let's use this JSON file instead, and make GetTor more robust.