Make the signing key downloaded by the Upgrader smaller
Originally created by @intrigeri on #17714 (Redmine)
As a follow-up to #15279 (closed) and cced26b6, it bugs me a little bit that the key downloaded by every Tails unnecessarily includes revoked subkeys.
Feature Branch: bugfix/17714-smaller-upgrader-signing-key+force-all-tests
Edited by intrigeri