Replace Dragon with a new machine

We recently had performance issues in Dragon (#18215 (closed), #18231), which resulted in the need to lower the number of Jenkins workers, degrading CI performance. The machine is also somewhat old (as is Iguana) and we're worried it can deteriorate further.

Note that we also need to replace Lizard (#18218), so we may take this opportunity to review the whole Tails fleet.

Ressources:

• see specs/constraints in this thread: #18215 (comment 3234922)
• see the research that lead to the statu quo, lots of this is still valid: https://gitlab.tails.boum.org/tails/blueprints/-/wikis/hardware_for_automated_tests_take3