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

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)

Related issues

• Blocked by #5906 (closed)
• Blocked by #6450 (closed)
• Blocked by #6458 (closed)

Original created by @tails on 5754 (Redmine)