Add 1.29 deprecation info for flowcontrol v1beta2

Author: liggitt

content/en/docs/reference/using-api/deprecation-guide.md

@@ -20,6 +20,20 @@ deprecated API versions to newer and more stable API versions.
 
 ## Removed APIs by release
 
+### v1.29
+
+The **v1.29** release will stop serving the following deprecated API versions:
+
+#### Flow control resources {#flowcontrol-resources-v129}
+
+The **flowcontrol.apiserver.k8s.io/v1beta2** API version of FlowSchema and PriorityLevelConfiguration will no longer be served in v1.29.
+
+* Migrate manifests and API clients to use the **flowcontrol.apiserver.k8s.io/v1beta3** API version, available since v1.26.
+* All existing persisted objects are accessible via the new API
+* Notable changes in **flowcontrol.apiserver.k8s.io/v1beta3**:
+  * The PriorityLevelConfiguration `spec.limited.assuredConcurrencyShares` field is renamed to `spec.limited.nominalConcurrencyShares`
+  * PriorityLevelConfiguration added `spec.limited.lendablePercent` and `spec.limited.borrowingLimitPercent` fields to control borrowing between levels
+
 ### v1.27
 
 The **v1.27** release will stop serving the following deprecated API versions: