Forward port release notes for v8.18.8 (elastic#137234)

Author: elasticsearchmachine

docs/reference/release-notes/8.18.8.asciidoc

@@ -1,10 +1,14 @@
 [[release-notes-8.18.8]]
 == {es} version 8.18.8
 
-coming[8.18.8]
-
 Also see <<breaking-changes-8.18,Breaking changes in 8.18>>.
 
+[IMPORTANT]
+====
+The 8.18.8 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-8.18.8]]
 [float]
 === Bug fixes