AWS RDS Database Version is different than Terraform version
AWS use default setting auto_minor_version_upgrade=true and tries to update your database.
You can do following to solve it
Method 1
Set flag to false explicitly using auto_minor_version_upgrade = false
Method 2
Use only first octet in version number engine_version = "11"
For more information https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance#engine_version