Change `ActivityPub::DeliveryWorker` retries to be spread out more (#21956)

app/workers/activitypub/delivery_worker.rb

@@ -10,6 +10,16 @@ class ActivityPub::DeliveryWorker
 
   sidekiq_options queue: 'push', retry: 16, dead: false
 
+  # Unfortunately, we cannot control Sidekiq's jitter, so add our own
+  sidekiq_retry_in do |count|
+    # This is Sidekiq's default delay
+    delay  = (count**4) + 15
+    # Our custom jitter, that will be added to Sidekiq's built-in one.
+    # Sidekiq's built-in jitter is `rand(10) * (count + 1)`
+    jitter = rand(0.5 * (count**4))
+    delay + jitter
+  end
+
   HEADERS = { 'Content-Type' => 'application/activity+json' }.freeze
 
   def perform(json, source_account_id, inbox_url, options = {})