Update changelog for v3.21.3 release
## Unreleased
+## 3.21.3 (September 22, 2022)
+
- Fix Helm Chart preview with unconfigured provider (C#) (https://github.com/pulumi/pulumi-kubernetes/issues/2162)
- Load default kubeconfig if not specified in provider (https://github.com/pulumi/pulumi-kubernetes/issues/2180)
- Skip computing a preview for Patch resources (https://github.com/pulumi/pulumi-kubernetes/issues/2182)
