Add frozen_string_literal to migration templates

Part of #47424

generator_templates/active_record/migration/create_table_migration.rb

+# frozen_string_literal: true
+
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 

generator_templates/active_record/migration/migration.rb

+# frozen_string_literal: true
+
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 

generator_templates/rails/post_deployment_migration/migration.rb

+# frozen_string_literal: true
+
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.