Deal with our website switching X.509 certificate authority

Originally created by @intrigeri on #8404 (Redmine)

Our website switched to a new SSL certificate after the release of 1.2.2. This new certificate is signed by a different CA. Tails Upgrader pins the expected CA. We prepared 1.2.2 to update Tails Upgrade to the new CA.

Feature Branch: bugfix/handle-website-CA-change

Related issues

Related to #8503 (closed)
Has duplicate #8494 (closed)

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