Tails Cloner should not upgrade the system partition if it's already up-to-date

Right now, we force a full reinstall whenever the target USB stick already has Tails and "Clone the current Persistent Storage" is checked.

That means:

• Reformatting
• Reinstalling the system partition
• Doing a full backup of the Persistent Storage

That's why we introduced the UI limitations in this case and display "You can only choose to reinstall when cloning the Persistent Storage" and make the "Upgrade" button unavailable. That's super weird. See our own confusion on #20247 (closed).

Whenever someone already has a Tails backup and wants to update it (let's say every week), we should instead propose to upgrade the backup and leave the partition table and system partition alone.

This is blocked by #8862.