add security advisory (#136019)

This PR adds security advisory for the 8.18.8 release notes.

Author: charlotte-hoblik

docs/reference/release-notes/8.19.5.asciidoc

@@ -5,6 +5,12 @@ coming[8.19.5]
 
 Also see <<breaking-changes-8.19,Breaking changes in 8.19>>.
 
+[IMPORTANT]
+====
+The 8.19.5 release contains fixes for potential security vulnerabilities.
+Please see our https://discuss.elastic.co/c/announcements/security-announcements/31[security advisory] for more details.
+====
+
 [[bug-8.19.5]]
 [float]
 === Bug fixes