Update Postgres terraform module to fix deprecation woarning

[jwalgran]

Terraform apply operations produce this deprecation warning

╷
│ Warning: Argument is deprecated
│
│   with module.database_enc.aws_db_instance.postgresql,
│   on .terraform/modules/database_enc/main.tf line 55, in resource "aws_db_instance" "postgresql":
│   55:   name                            = var.database_name
│
│ Use db_name instead
╵```

There is an issue to fix this in the upstream module

https://github.com/azavea/terraform-aws-postgresql-rds/issues/45

When this is fixed, and a new release is available, update to it.

[jwalgran]

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