Fix followers synchronization mechanism not being triggered on mentions (#15026)

e.g. if someone on an instance that previously had followers gets mentioned
in a private toot, before this PR, they would not receive a
Collection-Synchronization header and may show the toot to the former followers
in addition to the mentioned person.

app/services/process_mentions_service.rb

@@ -60,7 +60,7 @@ class ProcessMentionsService < BaseService
     if mentioned_account.local?
       LocalNotificationWorker.perform_async(mentioned_account.id, mention.id, mention.class.name, :mention)
     elsif mentioned_account.activitypub?
-      ActivityPub::DeliveryWorker.perform_async(activitypub_json, mention.status.account_id, mentioned_account.inbox_url)
+      ActivityPub::DeliveryWorker.perform_async(activitypub_json, mention.status.account_id, mentioned_account.inbox_url, { synchronize_followers: !mention.status.distributable? })
     end
   end