Merge pull request rails#53684 from flavorjones/flavorjones-fix-abort-if-pending-migrations

Fix task `abort_if_pending_migrations` to not use the return value from `Array#flatten!`

Author: kamipo

activerecord/lib/active_record/railties/databases.rake

@@ -333,7 +333,7 @@ db_namespace = namespace :db do
       pending_migrations << pool.migration_context.open.pending_migrations
     end
 
-    pending_migrations = pending_migrations.flatten!
+    pending_migrations.flatten!
 
     if pending_migrations.any?
       puts "You have #{pending_migrations.size} pending #{pending_migrations.size > 1 ? 'migrations:' : 'migration:'}"