Update CHANGELOG-EE for 8.8.3

[ci skip]

CHANGELOG-EE

@@ -3,9 +3,12 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.9.0 (unreleased)
 
 v 8.8.3
-  - Gracefully handle malformed DNs in LDAP group sync
-  - Make it clear the license overusage is visible only to admins
-  - Reduce load on DB for license upgrade check
+  - Add standard web hook headers to Jenkins CI post. !374
+  - Gracefully handle malformed DNs in LDAP group sync. !392
+  - Reduce load on DB for license upgrade check. !421
+  - Make it clear the license overusage message is visible only to admins. !423
+  - Fix Git hook validations for fast-forward merges. !427
+  - [Elastic] In search results, only show notes on confidential issues that the user has access to.
 
 v 8.8.2
   - Fix repository mirror updates for new imports stuck in started