Clean up the remainers of the old mirror pool setup

have all mirror operators disable the old VirtualHost
- update contribute/how/mirror doc to not document the old setup anymore (but still, keep dl.a.b.o working since that’s what the fallback DNS pool still uses)

Feature Branch: 451f:tails/doc/8643+mirrors

Original created by @intrigeri on 8643 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information