chore(ci): upgrade Helm to v3.20.0 for helm-unittest compatibility

Author: semih702t2t

.github/workflows/helm-regression.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: Setup Helm
         uses: azure/setup-helm@v4
         with:
-          version: v3.15.4
+          version: v3.20.0
 
       - name: Helm lint
         run: |