Have the Upgrader use the new mirror pool design
_Originally created by @intrigeri on [#11123 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/11123)_
The status quo, if we don’t do anything, would be that we go on pointing
to <http://dl.amnesia.boum.org/> in UDFs, and then all IUK downloads
will be pulled from the remaining DNS round-robin pool. Is it
acceptable, or do we want the Upgrader to use the new mirror pool?
Blueprint: https://tails.boum.org/blueprint/HTTP_mirror_pool/
Feature Branch: {tails,iuk,perl5lib}:feature/11123-new-mirror-pool
Parent Task: tails/tails#7161
### Related issues
- **Blocks** tails/sysadmin#11284
issue