(2) Finalize bigint conversion for deployment_merge_requests

deployment_merge_requests

Sql structure: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45316/commands/138975

Columns need to convert: [deployment_id, merge_request_id, environment_id] - Surprisingly project_id is not included here

1. Initialize & create bigint columns (done already by Tiger)
   1. Initialize conversion of deployment_merge_reque... (!195237 - merged)
2. Prepare Async Indexes - (TBD)
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)