Do 8 concurrent tasks instead of 10

This splits the 22 sets of images up more evenly, and uses a bit fewer
concurrent resources. Previous iteration overwhelmed the staging machine
into a memory allocation crash. Resources have also been increased.

app/services/card_image_loading_service.rb

@@ -17,7 +17,7 @@ def initialize(logger: nil, suppress_progress: false)
   def import
     barrier = Async::Barrier.new
     Sync do
-      semaphore = Async::Semaphore.new(10, parent: barrier)
+      semaphore = Async::Semaphore.new(8, parent: barrier)
 
       (1..22).map do |disk|
         semaphore.async do