Optimize map { ... }.compact calls (#15513)

* Optimize map { ... }.compact

using Enumerable#filter_map, supported since Ruby 2.7

* Add poyfill for Enumerable#filter_map

app/services/activitypub/process_collection_service.rb

@@ -37,7 +37,7 @@ class ActivityPub::ProcessCollectionService < BaseService
   end
 
   def process_items(items)
-    items.reverse_each.map { |item| process_item(item) }.compact
+    items.reverse_each.filter_map { |item| process_item(item) }
   end
 
   def supported_context?