fix(severity): add enum for severity values (#5856)

prowler/providers/aws/services/ec2/lib/instance.py

@@ -1,3 +1,4 @@
+from prowler.lib.check.models import Severity
 from prowler.providers.aws.services.ec2.ec2_service import Instance
 from prowler.providers.aws.services.vpc.vpc_service import VpcSubnet
 
@@ -15,13 +16,13 @@ def get_instance_public_status(
         tuple: The status and severity of the instance status.
     """
     status = f"Instance {instance.id} has {service} exposed to 0.0.0.0/0 but with no public IP address."
-    severity = "medium"
+    severity = Severity.medium
 
     if instance.public_ip:
         status = f"Instance {instance.id} has {service} exposed to 0.0.0.0/0 on public IP address {instance.public_ip} but it is placed in a private subnet {instance.subnet_id}."
-        severity = "high"
+        severity = Severity.high
         if vpc_subnets[instance.subnet_id].public:
             status = f"Instance {instance.id} has {service} exposed to 0.0.0.0/0 on public IP address {instance.public_ip} in public subnet {instance.subnet_id}."
-            severity = "critical"
+            severity = Severity.critical
 
     return status, severity