Update changelog for v2.8.4 release
## HEAD (Unreleased)
+## 2.8.4 (March 29, 2021)
+
- Ensure using `PULUMI_KUBERNETES_MANAGED_BY_LABEL` doesn't cause diffs on further stack updates (https://github.com/pulumi/pulumi-kubernetes/pull/1508)
- [sdk/ts] Update CustomResource to match current codegen (https://github.com/pulumi/pulumi-kubernetes/pull/1510)
