Update changelog for v2.9.1
## HEAD (Unreleased)
+## 2.9.1 (April 12, 2021)
+
- Fix refresh for render-yaml resources (https://github.com/pulumi/pulumi-kubernetes/pull/1523)
- Behavior change: Error on refresh for an unreachable cluster. (https://github.com/pulumi/pulumi-kubernetes/pull/1522)
