Adjust code that hardcodes our old signing key
Originally created by @intrigeri on #8738 (Redmine)
At least:
config/amnesia
config/chroot_local-includes/lib/live/config/2000-import-gnupg-key
features/step_definitions/checks.rb
Possibly some of this needs to be postponed to 1.3.1.
Parent Task: #8739 (closed)