AWS: DualStack VPC and DB Subnet Group

Hello Team,
We are trying to create a VPC on AWS, and though VPC and all networking is working, the DB Subnet Group shows that the Supported Network type as — or IPv4.

Is there any way to support it DualStack? I am attaching my Terraform Code and Variable Data, please let me know in case I am missing out on something!

Ref Code:

If I use the above config for VPC Generation via AWS Console, then If I generate RDS Subnet Group, it generates it with Supported Network as IPv4 and DualStack.

What am I missing in my terraform config?