Document the workflow to upgrade Tails from ISO using 2 USB sticks

Originally created by @sajolida on #6469 (Redmine)

It’s probably worth documenting the actual workflow of upgrading Tails from an ISO image as this most probably involves two USB sticks or more and a bit of gymnastics. All the concrete steps and requirements are already documented but explaining the workflow itself might help some people.

I basically see three options that are worth being mentioned:

1 - Using a DVD burner

• Burn the DVD with the new version.
• Clone and Upgrade onto Tails USB sticks.

2 - Using 3 USB sticks and whatever OS

• Have a “production” stick and an “testing” stick.
• When there is an upgrade, copy the ISO image onto a third USB stick.
• Start Tails on the “testing” stick (with the old version) and use “Upgrade from ISO” from the Tails Installer to install the new ISO image onto the “production” stick.

3 - Using 2 USB sticks and Linux

• Have a “production” stick and an “testing” stick.
• When there is an upgrade, copy the ISO image from my usual operating system into the persistent volume of the “testing” stick. If you use Linux it should offer you to open it with its passphrase.
• Start Tails on the “testing” stick (with the old version) and use “Upgrade from ISO” from the Tails Installer to install the new ISO image onto the “production” stick.

Related issues

• Related to #8581 (closed)