Add changelog entry for 8.15.9

Author: protectionsmachine

packages/security_detection_engine/changelog.yml

@@ -4,7 +4,7 @@
   changes:
     - description: Release security rules update
       type: enhancement
-      link: https://github.com/elastic/integrations/pulls/0000
+      link: https://github.com/elastic/integrations/pull/11549
 - version: 8.15.9-beta.1
   changes:
     - description: Release security rules update