Fix argument type for background migration

What does this MR do?

Follow-up to !49463 (merged), main issue #290008 (closed).

I got the argument type wrong, I was expecting an array of project IDs but missed that migrate_in uses splat to pass in each ID as its own argument.

Tested locally by:

1. Creating some duplicate records.
2. Running the new post migration in db/post_migrate/20210112143418_remove_duplicate_services2.rb.
3. Watching the BackgroundMigration jobs being processed in Sidekiq.
4. Verifying that the duplicate records were removed, and others kept.

Migration output

$ rails db:migrate:up VERSION=20201207165956
== 20201207165956 RemoveDuplicateServices: migrating ==========================
== 20201207165956 RemoveDuplicateServices: migrated (0.0000s) =================

$ rails db:migrate:down VERSION=20201207165956
== 20201207165956 RemoveDuplicateServices: reverting ==========================
== 20201207165956 RemoveDuplicateServices: reverted (0.0000s) =================

$ rails db:migrate:up VERSION=20210112143418
== 20210112143418 RemoveDuplicateServices2: migrating =========================
== 20210112143418 RemoveDuplicateServices2: migrated (0.0069s) ================

$ rails db:migrate:down VERSION=20210112143418
== 20210112143418 RemoveDuplicateServices2: reverting =========================
== 20210112143418 RemoveDuplicateServices2: reverted (0.0000s) ================

Does this MR meet the acceptance criteria?

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• [-] Tested in all supported browsers
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