update gate removal timeline

Author: kevindelgado

keps/sig-api-machinery/2885-server-side-unknown-field-validation/README.md

@@ -762,7 +762,8 @@ Pick one of these and delete the rest.
 - [x] Feature gate (also fill in values in `kep.yaml`)
   - Feature gate name: ServerSideFieldValidation
   - Components depending on the feature gate: kube-apiserver
-  - Note: feature gate removed upon graduation to GA (1.27)
+  - Note: feature gate locked to true upon graduation to GA (1.27) and removed
+    two releases later (1.29)
 - [x] Other
   - Describe the mechanism: query parameter
   - Will enabling / disabling the feature require downtime of the control