CHANGELOG: add decommissioned flags

To update the changelog with the deprecated flags that re decommissioned in v3.6.

Author: Niket1997

CHANGELOG/CHANGELOG-3.6.md

@@ -35,6 +35,7 @@ In this release, we have removed several flags that were available in etcd 3.5.
 - `--proxy-dial-timeout`
 - `--proxy-write-timeout`
 - `--proxy-read-timeout`
+
 <hr>
 
 ## v3.6.0-rc.1 (2025-02-25)