doc: fix v16.17.1 security release changelog

doc/changelogs/CHANGELOG_V16.md is missing the security release header
prior to the "Notable changes" section.

PR-URL: #44759
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Beth Griggs <bethanyngriggs@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: ruyadorno

doc/changelogs/CHANGELOG_V16.md

@@ -68,6 +68,8 @@
 
 ## 2022-09-23, Version 16.17.1 'Gallium' (LTS), @ruyadorno
 
+This is a security release.
+
 ### Notable changes
 
 The following CVEs are fixed in this release: