Update changelog for 2.7.6 release
## HEAD (Unreleased)
+## 2.7.6 (January 13, 2021)
+
- Upgrade helm and k8s deps (https://github.com/pulumi/pulumi-kubernetes/pull/1414)
- Fix bug with Go Helm v3 transformation marshaling (https://github.com/pulumi/pulumi-kubernetes/pull/1420)
