refactor(security): replace generic event with severity enum

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

core/lib/Security/Event/AccessDeniedEvent.php

@@ -81,8 +81,8 @@ final class AccessDeniedEvent extends Event implements SecurityRequestEventInter
         return $this->reason;
     }
 
-    public function getSeverity(): int
+    public function getSeverity(): SecurityEventSeverity
     {
-        return SecurityEvent::SEVERITY_WARNING;
+        return SecurityEventSeverity::WARNING;
     }
 }