Fix NoMethodError for ActiveRecord 7.1.2 in ShortenerGenerator

Fixes error: 

`method_missing': undefined method `timestamped_migrations' for ActiveRecord::Base:Class (NoMethodError)
	from ~/.rvm/gems/ruby-3.2.2@maitre-d/gems/shortener-1.0.1/lib/generators/shortener/shortener_generator.rb:12:in `next_migration_number'

lib/generators/shortener/shortener_generator.rb

@@ -9,7 +9,7 @@ def self.source_root
   end
 
   def self.next_migration_number(dirname)
-    if ActiveRecord::Base.timestamped_migrations
+    if ActiveRecord.timestamped_migrations
       Time.new.utc.strftime("%Y%m%d%H%M%S")
     else
       "%.3d" % (current_migration_number(dirname) + 1)