Update changelog for v2.8.1 release
## HEAD (Unreleased)
+## 2.8.1 (February 12, 2021)
+
- Skip Helm test hook resources by default (https://github.com/pulumi/pulumi-kubernetes/pull/1467)
- Ensure no panic when a kubernetes provider is used with an incompatible resource type (https://github.com/pulumi/pulumi-kubernetes/pull/1469)
- Allow users to set `PULUMI_KUBERNETES_MANAGED_BY_LABEL` environment variable to control `app.kubernetes.io/managed-by` label (https://github.com/pulumi/pulumi-kubernetes/pull/1471)
