Upgrade AMD processor microcodes to mitigate the Spectre attack

• Intel: https://tracker.debian.org/pkg/intel-microcode, https://bugs.debian.org/886367
  • AMD: https://tracker.debian.org/pkg/amd64-microcode, https://bugs.debian.org/886382

Feature Branch: bugfix/15148-cpu-microcode-spectre

Related issues

• Related to #14976 (closed)
• Blocks #13245 (closed)
• Blocks #15173 (closed)

Original created by @intrigeri on 15148 (Redmine)