Fix existing boosts possibly referencing deleted toots (#17730)

Follow-up to #17693

db/migrate/20220309213005_fix_reblog_deleted_at.rb

@@ -0,0 +1,9 @@
+class FixReblogDeletedAt < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def up
+    safety_assured { execute 'UPDATE statuses s SET deleted_at = r.deleted_at FROM statuses r WHERE s.reblog_of_id = r.id AND r.deleted_at IS NOT NULL' }
+  end
+
+  def down; end
+end