Ensure event subscriptions are enabled for instance level events

Event subscriptions use Simple Notification Service (Amazon SNS) to send notifications which can help ensure high-priority events are addressed in a timely manner. For more information, see the AWS documentation.
References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Events.html

In AWS Console -

1. Sign in to the AWS Console and go to the Amazon RDS console.
2. Select Event subscriptions in navigation pane.
3. Select Create event subscription in the Event subscriptions pane.
4. In the Create event subscription dialog box, do the following:
   a. Enter a name for the event notification subscription.
   b. Select an existing Amazon SNS ARN for an Amazon SNS topic, or select create topic to select an existing topic and a list of recipients.
   c. Select a source type.
   d. Select Yes to enable the subscription.
   e.Select the event categories and sources that you want to receive event notifications for.
   f.Click Create.

In Terraform -

1. In an aws_db_event_subscription resource, set source_type to db-security-group.

References:
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_event_subscription#source_type