Update changelog for v3.20.0 release
## Unreleased
+## 3.20.0 (July 12, 2022)
+
- Implement Server-Side Apply mode (https://github.com/pulumi/pulumi-kubernetes/pull/2029)
- Add Patch resources to all SDKs (https://github.com/pulumi/pulumi-kubernetes/pull/2043) (https://github.com/pulumi/pulumi-kubernetes/pull/2068)
- Add awaiter for service-account-token secret (https://github.com/pulumi/pulumi-kubernetes/pull/2048)
