Announce available upgrades with Tails Upgrader rather than with tails-security-check

Originally created by Tails on #5754 (Redmine)

The idea behind the proposed implementation is that:

  • Tails Upgrader announces available upgrades (security issues marked as fixed)
  • tails-security-check announces security issues that are not marked as fixed

Feature Branch: feature/incremental-upgrades-integration

Parent Task: #6014 (closed)

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