Update changelog for v3.20.3 release
## Unreleased
+## 3.20.3 (August 9, 2022)
+
- Add chart v2 deprecation note to schema/docs (https://github.com/pulumi/pulumi-kubernetes/pull/2114)
- Add a descriptive message for an invalid Patch delete (https://github.com/pulumi/pulumi-kubernetes/pull/2111)
- Fix erroneous resourceVersion diff for CRDs managed with SSA (https://github.com/pulumi/pulumi-kubernetes/pull/2121)
