Fleet Server / Elastic Agent 9.1.2 release notes (#2576)

Adds a release notes entry for release 9.1.2. Although no changes were
made in Fleet and Elastic Agent between 9.1.1 and 9.1.2, we still need
to add an entry specifying this as customers can upgrade to this
version.

cc @ebeahan @pierrehilbert

---------

Co-authored-by: Colleen McGinnis <[email protected]>

Author: vishaangelova

release-notes/fleet-elastic-agent/index.md

@@ -26,6 +26,10 @@ To check for security updates, go to [Security announcements for the Elastic sta
 % ### Fixes [fleet-elastic-agent-next-fixes]
 % *
 
+## 9.1.2 [fleet-elastic-agent-9.1.2-release-notes]
+
+There are no new features, enhancements, or fixes in this release.
+
 ## 9.1.1 [fleet-elastic-agent-9.1.1-release-notes]
 
 ### Features and enhancements [fleet-elastic-agent-9.1.1-features-enhancements]