Upgrade automatic and manual upgrade screenshots

Once 3.1 is out:

We should upgrade the automatic upgrade screenshot in /doc/first_steps/upgrade.
We could add another one for manual upgrade so that both contexts are clarified.

But let’s not bother doing that before 3.0 because it would be too painful to be worth it.

Feature Branch: spriver:web/12365-update-upgrader-screenshots

Original created by @sajolida on 12365 (Redmine)

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