chore: CHANGELOG for 1.2.1 (#3480)

Signed-off-by: Mattia Lavacca <[email protected]>
Co-authored-by: Mattia Lavacca <[email protected]>

Author: k8s-infra-cherrypick-robot

CHANGELOG/1.2-CHANGELOG.md

@@ -2,10 +2,24 @@
 
 ## Table of Contents
 
+- [v1.2.1](#v121)
 - [v1.2.0](#v120)
 - [v1.2.0-rc2](#v120-rc2)
 - [v1.2.0-rc1](#v120-rc1)
 
+# v1.2.1
+
+This is a patch release that fixes the backward incompatibility with the `SupportedFeatures`
+feature breaking change introduced in v1.2.1.
+
+## Changes by Kind
+
+### Bug Fixes
+
+- `v1.2.0` introduced a breaking change in the `SupportedFeatures` field of the `GatewayClass`
+  API. That broke already existing `GatewayClass`es using the previous version of
+  the feature. The fix to introduce backward compatibility is in (#3454, @LiorLieberman)
+
 # v1.2.0
 
 ## Breaking Changes