release_process: update transmission removal instructions

Originally created by @CyrilBrulebois on #16593 (Redmine)

Current post-release instructions:

1. If you just released a new stable release, remove the previous
   stable release from:
   - our rsync server:
     `ssh rsync.lizard rm -rf /srv/rsync/tails/tails/stable/tails-amd64-${PREVIOUS_VERSION:?}/`
   - our Bittorrent seed: get the previous release's _Transmission_ ID
     with `ssh bittorrent.lizard transmission-remote --list` and then
     delete it with
     `ssh bittorrent.lizard transmission-remote -t "${PREVIOUS_VERSION_TRANSMISSION_ID:?}" --remove-and-delete`

Now that we ship ISO+IMG images, we have to look up two IDs. FWIW, I had 1-2 for 3.12.1 ISO+IMG (or IMG+ISO…) and 3-4 for 3.13 ISO+IMG (or IMG+ISO…), and could remove the first two by passing the -t 1,2 option.