(2) Finalize bigint conversion for deployment_clusters

deployment_clusters

Sql structure: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45316/commands/138970

Columns need to convert: [deployment_id, cluster_id]

1. Initialize & create bigint columns (done already by Tiger)
   1. Initialize conversion of deployment_clusters to... (!195236 - merged)
2. Prepare Async Indexes
   1. I don't think we need this, the table size is only 2GB and far smaller than the other two
3. (the following week) Add indexes normally, add NOT VALID foreign keys, prepare async foreign key validation
4. (the following week after 3) Perform the swap, remove old indexes and foreign keys (async)