Refactor security configuration: Remove external CrowdSec mode support

- Updated SecurityConfig model to only support 'local' or 'disabled' modes for CrowdSec.
- Modified related logic in the manager and services to reject external mode.
- Adjusted tests to validate the new restrictions on CrowdSec modes.
- Updated frontend components to remove references to external mode and provide appropriate user feedback.
- Enhanced documentation to reflect the removal of external CrowdSec mode support.

frontend/src/api/security.ts

@@ -3,7 +3,7 @@ import client from './client'
 export interface SecurityStatus {
   cerberus?: { enabled: boolean }
   crowdsec: {
-    mode: 'disabled' | 'local' | 'external'
+    mode: 'disabled' | 'local'
     api_url: string
     enabled: boolean
   }