Update check-mirrors to support the new mirror pool design

It’ll need to deal with the fact tha will will actually two (possibly intersecting?) pools: the remaining DNS one, and the new one described on the blueprint.

Blueprint: https://tails.boum.org/blueprint/HTTP_mirror_pool/#configuration

Feature Branch: check-mirrors:master

Parent Task: #7161 (closed)

Original created by @intrigeri on 11122 (Redmine)