Enable frozen string for some vestigial app/ files

Turns out false satisfies the cop as well.

xref:

https://github.com/rubocop-hq/rubocop/issues/7241
https://github.com/rubocop-hq/rubocop/issues/7426

https://gitlab.com/gitlab-org/gitlab-foss/issues/47424
https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/25432

app/workers/migrate_external_diffs_worker.rb

-# frozen_string_literal: false
+# frozen_string_literal: true
 
 class MigrateExternalDiffsWorker
   include ApplicationWorker

app/workers/schedule_migrate_external_diffs_worker.rb

-# frozen_string_literal: false
+# frozen_string_literal: true
 
 class ScheduleMigrateExternalDiffsWorker
   include ApplicationWorker

app/workers/update_project_statistics_worker.rb

-
 # frozen_string_literal: true
 
 # Worker for updating project statistics.