Use `supports_severity?` to check feature availability on issuables

Avoid checking issue type.

app/models/concerns/issuable.rb

@@ -197,7 +197,7 @@ module Issuable
     end
 
     def severity
-      return IssuableSeverity::DEFAULT unless incident?
+      return IssuableSeverity::DEFAULT unless supports_severity?
 
       issuable_severity&.severity || IssuableSeverity::DEFAULT
     end

app/services/incident_management/incidents/update_severity_service.rb

@@ -12,7 +12,7 @@ module IncidentManagement
       end
 
       def execute
-        return unless issuable.incident?
+        return unless issuable.supports_severity?
 
         update_severity!
         add_system_note