Adapt tools/update-responses/update_responses to support multiple versions in the same xml files

For tor-browser#42747 (closed), the generated update_responses xml files will need to include two versions: the normal branch and the legacy 13.5 branch.

tools/update-responses/update_responses currently only supports having one version, so needs to be updated to support that.

At the same time we can drop the sha512 hash since it's not checked anymore (tor-browser#42737 (closed)) and we're going to have a watershed update so support for update from older versions won't be needed.