incremental upgrades: complete phase three

Originally created by Tails on #6014 (Redmine)

This is a placeholder for what we think should follow phase two and is needed to enable the automatic update process by default in the next Tails release.

Feature Branch: feature/incremental-upgrades-integration

Parent Task: #5919 (closed)

Subtasks

• #5906 (closed)
• #6449 (closed)
• #6450 (closed)
• #5754 (closed)
• #6448 (closed)
• #6451 (closed)
• #6458 (closed)
• #6495 (closed)
• #6503 (closed)
• #6506 (closed)
• #6501 (closed)
• #6509 (closed)
• #6511 (closed)
• #6535 (closed)

Related issues

• Related to #6489 (closed)
• Blocked by #5490 (closed)
• Blocks #6508 (closed)
• Blocked by #5735 (closed)