Prepare conversion of abuse_reports to bigint.

What does this MR do and why?

Prepares conversion of abuse_reports to bigint for assignee_id, id, reporter_id, resolved_by_id, user_id.

You can read more about the process for preparing bigint conversion in https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#migrating-integer-primary-keys-to-bigint.

As part of our process we want to ensure all ID columns are bigint to avoid the risk of overflowing while we continue our growth.

See Convert all integer IDs to bigint in the primar... (#465805 - closed)

Verify this MR as it was automatically created by gitlab-housekeeper.

Ensure that those columns are not being converted yet in the production database by checking Joe Bot through https://console.postgres.ai/gitlab. If the columns were already converted in another merge request, consider closing this merge request.

This change was generated by gitlab-housekeeper using the Keeps::InitializeBigIntConversion keep.

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Changelog: added

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