Stop shipping our old signing key in the ISO
Originally created by @intrigeri on #8735 (Redmine)
We can do that in 1.3.1, since we’ll be in the final stage of the transition to our new signing key then. We should do that in 1.3.1, as we want Tails Upgrader to stop trusting the old signing key at that point.
Parent Task: #8740 (closed)