Prepare changelog for v3.20.4
## Unreleased
+## 3.20.4 (August 15, 2022)
+
- Fix Helm charts being ignored by policy packs. (https://github.com/pulumi/pulumi-kubernetes/pull/2133)
- Fixes to allow import of helm release (https://github.com/pulumi/pulumi-kubernetes/pull/2136)
- Keep managed-by label in SSA mode if already present (https://github.com/pulumi/pulumi-kubernetes/pull/2138)
