Remove AddTrust_External_Root.pem from our website CA bundle

In the Tails release that follows our website’s switch to Let’s Encrypt -issued certificates, we can have Tails Upgrader & friends stop trusting the CA previously used by our website.

Feature Branch: feature/11811-remove-AddTrust-CA-from-our-website-bundle

Parent Task: #11809 (closed)

Original created by @intrigeri on 11811 (Redmine)

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