Update changelog for 1.9 release in progress. (#4137)

Signed-off-by: Goutham Veeramachaneni <[email protected]>

Author: gouthamve

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## master / unreleased
 
+## 1.9.0 in progress
+
 * [CHANGE] Fix for CVE-2021-31232: Local file disclosure vulnerability when `-experimental.alertmanager.enable-api` is used. The HTTP basic auth `password_file` can be used as an attack vector to send any file content via a webhook. The alertmanager templates can be used as an attack vector to send any file content because the alertmanager can load any text file specified in the templates list. #4129
 * [CHANGE] Alertmanager now removes local files after Alertmanager is no longer running for removed or resharded user. #3910
 * [CHANGE] Alertmanager now stores local files in per-tenant folders. Files stored by Alertmanager previously are migrated to new hierarchy. Support for this migration will be removed in Cortex 1.11. #3910