rds: add db.m6idn instance family (#591)

Author: GeorgeSimpsonAtFanatics

rules/aws_db_instance_invalid_type.go

@@ -97,6 +97,15 @@ func NewAwsDBInstanceInvalidTypeRule() *AwsDBInstanceInvalidTypeRule {
 			"db.m6id.2xlarge":           true,
 			"db.m6id.xlarge":            true,
 			"db.m6id.large":             true,
+			"db.m6idn.large":            true,
+			"db.m6idn.xlarge":           true,
+			"db.m6idn.2xlarge":          true,
+			"db.m6idn.4xlarge":          true,
+			"db.m6idn.8xlarge":          true,
+			"db.m6idn.12xlarge":         true,
+			"db.m6idn.16xlarge":         true,
+			"db.m6idn.24xlarge":         true,
+			"db.m6idn.32xlarge":         true,
 			"db.m7g.16xlarge":           true,
 			"db.m7g.12xlarge":           true,
 			"db.m7g.8xlarge":            true,