Upgrade Intel processor microcodes to mitigate the Spectre attack

This is a follow-up on #15148 (closed) because we could not upgrade the Intel microcode in Tails 3.5.

Once https://bugs.debian.org/886998 is fixed we can revert eab2135464682cac54ed5cfc8ef2f9e0529a9913.

Feature Branch: feature/15173-upgrade-intel-microcode

Original created by @intrigeri on 15173 (Redmine)

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