Update changelog for v3.21.1 release
## Unreleased
+## 3.21.1 (August 31, 2022)
+
- Update Helm and Kubernetes module dependencies (https://github.com/pulumi/pulumi-kubernetes/pull/2152)
- Automatically fill in .Capabilities in Helm Charts (https://github.com/pulumi/pulumi-kubernetes/pull/2155)
