Publish Torrents for beta and RCs

As decided in #10915 (closed). Now we need to adjust the release process:

Our release process doc already makes us generate a Torrent and its
detached signature, even for RC:s in the “Generate the OpenPGP signatures and Torrents” which seems to have no condition attached. It also makes us seed this Torrent unconditionally.

But we need to change:

• in the “Update the website and Git repository” section: don’t skip
  the Torrent publication steps when preparing a RC; also deal with
  cleaning RC:s’ Torrent files later

Parent Task: #10781 (closed)

Original created by @sajolida on 11126 (Redmine)