Upgrader fails to check upgrades: CA changed for our website

This issue is about updating the pinned CAs in tails.git, so Tails 4.15 and newer have a working upgrader once the new certificate is put back in place. The upgrader should support both the old and new CAs for now, to allow a seamless transition.

The sysadmin part of it, to workaround this problem on Tails 4.14 and earlier (for a month, which is the best we can do), is sysadmin#17791 (closed).

Incidentally, this problem breaks most of our test suite.

• Remove the note on /doc/upgrade/error/check.
• Remove the note on /home.