Remove option_group_name so the RDS upgrade can apply

[ale210]

Fixes the failed apply from #152. Re-opens the work on #150.

What happened

#152 merged and terraform apply failed:

Error: updating RDS DB Instance (incubator-prod-database): operation error RDS:
ModifyDBInstance, StatusCode: 404, OptionGroupNotFoundFault:
Specified OptionGroupName: default:postgres-15 not found.

AWS creates default option groups lazily, exactly as it does default parameter groups. default:postgres-15 does not exist in this account, so naming it fails before RDS does any work.

No damage. The call was rejected before anything was modified. The instance is still 13.20, available, on its original parameter and option groups, with an unchanged endpoint. There was no downtime. aws_db_parameter_group.postgres15 was created successfully by that run and is already in state.

The change

Remove option_group_name from aws_db_instance.default rather than pointing it somewhere else.

Option groups do nothing for PostgreSQL — no options exist for the postgres engine — and the attribute is Optional+Computed in the AWS provider. With it absent, Terraform stops proposing changes to it and RDS assigns the postgres-15 family default itself during the major upgrade, creating default:postgres-15 lazily at that point. That is the one moment it can be created, which is why it cannot be named up front.

The alternative, an aws_db_option_group resource with major_engine_version = "15", also works but adds a managed resource whose only job is to hold zero options indefinitely.

A comment is included above the resource so the attribute does not get reintroduced.

Note for whoever refines #150

The action item "Change option_group_name from "default:postgres-13" to "default:postgres-15" — same family constraint" is incorrect, and is what caused the failed apply. The item should say to remove the attribute. The reasoning it gives for parameter groups (AWS creates defaults lazily) applies identically to option groups, but the ticket draws the opposite conclusion for them.

Before merging

Confirm the plan is an in-place update to aws_db_instance.default with 0 to destroy. skip_final_snapshot = true and deletion_protection = false mean a replace would destroy the shared database.

Merging is the deploy: apply_immediately = true, so the upgrade begins on merge. Expect roughly 10-20 minutes of downtime.

Prerequisites remain satisfied — snapshot incubator-prod-database-pre-pg15, PostGIS updated to 3.4.3 on both ballotnav databases, no replication slots, no reg* columns.

[github-actions]

Terraform plan in terraform
With backend config files: terraform/prod.backend.tfvars

Plan: 0 to add, 1 to change, 0 to destroy.

Terraform used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
!~  update in-place

Terraform will perform the following actions:

  # aws_db_instance.default will be updated in-place
!~  resource "aws_db_instance" "default" {
!~      engine_version                        = "13.20" -> "15"
        id                                    = "db-WEFHC5MZIA5NX5PBTQZPC2STZQ"
!~      parameter_group_name                  = "default.postgres13" -> "incubator-prod-postgres15"
        tags                                  = {
            "Name"              = "incubator-prod-database"
            "terraform_managed" = "true"
        }
#        (72 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

✅ Plan applied in Terraform apply (OIDC) #61